Exploring the natural progression of diseases from onset to cure as well as modes of disease transmission.
What is covered on this page →
Explain the dynamics of diseases.
Illustrate the interaction between host, agent, vector and the environment.
Describe the mode of disease transmission.
The Natural History of Diseases
Understanding the natural history of a disease is essential for healthcare professionals, epidemiologists, and researchers, as it informs strategies for prevention, early detection, and treatment. It helps identify critical points for intervention, such as vaccination, lifestyle modifications, or therapeutic interventions, to alter the disease’s course or mitigate its impact on individuals and populations. The natural history of disease is therefore a fundamental concept in epidemiology and public health that describes the progression of a disease within an individual or a population over time. In essence, it encompasses the entire lifecycle of the disease, from its initial causes and development to its clinical manifestations, possible complications, and outcomes, including recovery or death. There is a complex interplay from the initial exposure to the ultimate outcomes of the disease. This discussion will provide a robust overview of the natural history of disease, including its stages, factors influencing disease progression, and its relevance in public health.
Figure 2.1. The Temporal Course of Disease from Susceptibility to Resolution
The principles pertaining to the dynamics and natural history examines the diagnostic processes from the host-agent interaction through to the end of the disease process in which there is no form of intervention or treatment. There are four main phases with distinct characteristics however, these characteristics do not always define the timeline associated with each phase due to the overlapping nature of some of these characteristics. This flow diagram (Figure 2.2) provides further explanation of the characteristics of each phase depicted in Figure 2.1.
Figure 2.2. The Characteristics of each Phase in the Natural History of Diseases.
During the Introduction to Epidemiology discussion we mentioned that epidemiologists aim to understand, “what contributes to our well-being, what leads to illness, and what contributes to our recovery.” Therefore, we will examine some of the factors that increase our susceptibility to diseases as we explore the natural history of diseases.
The occurrence of diseases begin with a population or an individual being susceptible to contracting a particular disease. Susceptibility can be influenced by various factors, including genetics, age, sex, immune status, environmental exposures, and behaviors. We may also discuss susceptible for diseases in terms of host, agent, environmental, or behavioural factors.
Host factors give rise to variability in disease susceptibility. Diseases such as breast and ovarian cancer, familial hypercholesterolemia, and cardiovascular diseases are influenced by one’s genetic predisposition. Young children and older adults may be more susceptible to some infectious diseases. Men are more susceptible to heart disease, diabetes and cerebrovascular events while females are more susceptible to arthritis and depression. A total of 10-15% of individuals with depression may be at risk of suicide; however, men are approximately two-thirds more likely than women to die by suicide (WHO, 2021). The overall health status of individuals play a significant role in disease susceptibility and progression.
The characteristics of the disease-causing agent, including its virulence, mode of transmission, and mutation rate, significantly impact the natural history of the disease. For example, infections such as Marburg virus are easily transmitted from bats to humans during activities like mining. Subsequently, the disease can be readily transmitted from human to human, and it exhibits a high case fatality rate of 88% (CDC, 2021). Rabies encephalitis, another zoonotic viral disease, is transmitted to humans either by domestic animals like dogs and cats or by wild animals such as bats, skunks, and raccoons. This disease has an exceptionally high case fatality rate of approximately 100% (WHO, 2021). During the COVID-19 pandemic, a hallmark was the highly transmissible and rapidly mutating SARS-CoV-2 virus (CDC, 2021). It’s important to note that agent factors are complex and not limited to the information provided within the scope of this discussion.
Environmental Factors such as air pollution, particularly exposure to fine particulate matter (PM2.5) and ground-level ozone, is a significant environmental factor that can worsen asthma symptoms and increase susceptibility to other respiratory diseases. Individuals living in areas with high levels of air pollution are at an increased risk of developing asthma or experiencing exacerbations of pre-existing asthma. The fine particles and pollutants in the air can irritate the airways, trigger inflammation, and exacerbate asthma symptoms, making it harder for individuals to breathe. Prolonged exposure to poor air quality can lead to the development of chronic respiratory conditions (Gauderman, et al, 2004). Climate change, including temperature and precipitation patterns, can significantly influence the transmission of vector-borne diseases like malaria. Malaria transmission is highly sensitive to environmental conditions. An increase in temperature can expand the geographic range of malaria-carrying mosquitoes, allowing the disease to spread to new areas where it was previously rare. Changes in rainfall patterns can create breeding sites for mosquitoes, increasing their population and the risk of malaria transmission. Conversely, droughts can reduce water sources, limiting mosquito breeding. Therefore, alterations in climate can directly affect the prevalence and distribution of malaria (Caminade, et al, 2014).
Behavioral factors can significantly impact disease susceptibility by influencing an individual’s lifestyle choices and health-related practices. Tobacco smoking is a well-known behavioral risk factor that greatly increases susceptibility to lung cancer. Smoking introduces harmful chemicals into the lungs, leading to chronic irritation and inflammation. Over time, this chronic exposure can cause mutations in lung cells, leading to the development of cancer. Individuals who engage in smoking behaviors are at a much higher risk of developing lung cancer compared to non-smokers. Smoking is considered the leading cause of preventable deaths worldwide and is responsible for a significant portion of lung cancer cases (American Cancer Society, 2021). Another well-known example is, poor dietary choices characterized by the consumption of high-sugar, high-fat, and low-fiber foods. These can increase the risk of developing type 2 diabetes. A diet that is rich in sugary beverages, highly processed foods, and saturated fats can lead to obesity and insulin resistance, both of which are key contributors to type 2 diabetes. Excessive consumption of sugary foods and drinks can also lead to weight gain and increase the risk of developing metabolic syndrome. Individuals who consistently make unhealthy dietary choices are more susceptible to developing type 2 diabetes compared to those who maintain a balanced diet (Hu, 2011).
Three Stages of the Disease Process
- The Pathological Onset of disease
- Pre-symptomatic stage
- Clinical manifestation
The Pathological Onset of Disease
This stage represents the initial phase of the disease process, where changes at the cellular or molecular level begin to occur within the body. Pathological onset may be triggered by various factors such as infection, exposure to toxins, genetic mutations, or other environmental factors. During this stage, the disease process is often asymptomatic, meaning there are no noticeable symptoms or signs of illness. However, pathological changes are underway within the body.
Pre-symptomatic Stage of Disease
The pre-symptomatic stage occurs after the pathological onset but before the appearance of overt clinical symptoms. In this stage, the disease continues to progress, and specific pathological changes are taking place in affected tissues or organs. While the individual may not be experiencing symptoms at this stage, diagnostic tests or screenings may detect abnormalities indicative of the disease. Consider the onset of Type 2 Diabetes, the individual has no symptoms however, HbA1c testing may detect higher than normal percentage glycated haemoglobin or fasting blood glucose tests may reveal results higher than the upper limit of normal. This stage highlights the importance of early detection and intervention to manage the disease effectively, especially when treatment options are more successful in the early phases.
Clinical Manifestation of Disease
The clinical manifestation stage is when the disease becomes clinically apparent, and individuals experience recognizable symptoms and signs of illness. These symptoms can vary widely depending on the type and stage of the disease but often include pain, discomfort, fatigue, fever, or other characteristic signs. Healthcare providers can make a definitive diagnosis based on clinical manifestations and additional diagnostic tests, such as blood tests, imaging, or biopsies. Treatment and management strategies are typically initiated during this stage to alleviate symptoms, slow disease progression, or cure the condition if possible.
For instance, in the clinical manifestation stage of type 2 diabetes, individuals may experience symptoms such as frequent urination, excessive thirst, unexplained weight loss, and fatigue. A healthcare provider can diagnose the condition by assessing these clinical manifestations, conducting blood tests to measure glucose levels, and confirming the presence of high blood sugar. Once diagnosed, treatment may include lifestyle modifications, medications, and monitoring to manage the disease and prevent complications.
Importance of the Natural History of Diseases
The natural history of disease plays a pivotal role in healthcare, public health, research, resource allocation, patient empowerment, economic planning, and ethical considerations. It guides interventions, improves outcomes, and informs decisions at multiple levels of healthcare and society.
Understanding the natural history of diseases allows for early intervention and prevention. For instance, in the case of cervical cancer, knowing the natural history, which involves precancerous stages (such as cervical dysplasia) before the development of invasive cancer, has led to the implementation of regular Pap smear screening programs. Early detection and treatment during the precancerous stages can prevent the progression to invasive cancer, saving lives and reducing healthcare costs.
Healthcare resources are limited, and knowledge of disease natural history helps allocate them efficiently. For example, in managing diabetes, understanding that the disease progresses through stages from prediabetes to overt diabetes allows healthcare systems to prioritize interventions like lifestyle modifications and early pharmacological treatment for individuals at higher risk of progression. This targeted approach optimizes resource use.
Public health planning relies on the natural history of diseases. Consider the example of HIV/AIDS. Understanding the natural history of HIV infection, including the asymptomatic phase and progression to AIDS, has guided public health efforts. This knowledge has led to strategies such as widespread HIV testing, access to antiretroviral therapy, and educational campaigns to reduce transmission, significantly impacting the course of the epidemic.
In research and treatment development, the natural history of diseases informs drug and therapy targets. For instance, in cancer research, knowledge of how tumors progress from benign to malignant helps researchers identify molecular markers and pathways for potential targeted therapies. This understanding has led to the development of precision medicine approaches, tailoring treatments to the specific characteristics of a patient’s cancer.
Patients benefit from understanding the natural history of their condition. For example, in asthma management, patients who know that asthma exacerbations can often be prevented with proper maintenance medication adherence are more likely to engage in self-care and follow prescribed treatment plans. This knowledge empowers patients to take an active role in managing their health.
The natural history of disease has economic implications. Take the example of smoking and lung cancer. Understanding that smoking is a key risk factor for lung cancer and that it may take years or decades for lung cancer to develop has influenced public policies, such as tobacco taxation and smoking cessation programs. These measures aim to reduce both the economic burden of treating lung cancer and the societal costs associated with lost productivity due to illness and premature death.
Ethical decisions in healthcare are influenced by the natural history of disease. For instance, in advanced dementia, understanding the natural history helps families and healthcare providers make informed decisions about end-of-life care, including decisions about the use of life-sustaining treatments, hospice care, and advanced directives. This knowledge supports ethical and compassionate care for patients at the end of life.
The Epidemiologic Triad
Figure 2.3. The Epidemiologic Triangle
The epidemiologic triad is the traditional model of infectious disease causation, also known as the epidemiologic triangle, consists of three components: the host, agent, and the environment.
Host
The host refers to the individual or population at risk of developing the disease. It includes factors such as genetics, age, sex, immunity, and behaviors.
Agent
The agent represents the pathogen or factor that causes the disease. This can include bacteria, viruses, toxins, chemicals, or other causal agents.
Environment
The environment encompasses the external factors that influence the transmission of the disease. These factors can include physical surroundings, climate, socioeconomic conditions, and more.
The epidemiologic triad and the natural history of disease are interconnected in the context of disease transmission. The host-agent components of the triad are closely related to the natural history of disease. For example, the host’s susceptibility and immunity play a significant role in determining how the disease progresses within an individual. The agent, representing the pathogen or causal factor, is central to the natural history of the disease. The characteristics of the agent, including its virulence and mode of transmission, influence the stages of disease development. The environmental component of the triad can impact both the host and the agent. Environmental factors such as living conditions, sanitation, and climate can affect disease transmission and the natural history of the disease. Let us explore some examples looking at the fictitious city of Bridgetown with a high population density and approximately one-third of this population living below the poverty line:
Malaria in the Tropical city of Bridgetown
In Bridgetown, the residents, especially those living in poverty, are susceptible hosts for Malaria infection. Poverty can lead to inadequate housing conditions and limited access to healthcare, making individuals more vulnerable to infection. The agent in this case is the Plasmodium parasite, primarily transmitted by Anopheles mosquitoes. Since Bridgetown has a tropical climate, it provides a conducive environment for mosquito breeding. Factors such as stagnant water sources and poor sanitation contribute to mosquito proliferation.
An understanding of each component of the epidemiologic triad is essential for effective intervention. Mosquito (vector) control programs, such as insecticide-treated bed nets, indoor residual spraying, and eliminating breeding sites, can reduce mosquito populations. A more expansive in integrated approach would be, addressing poverty through social programs, better housing, and improved access to healthcare thereby reducing host susceptibility. Public health campaigns can educate residents about the importance of protective measures like using bed nets and seeking prompt medical care for symptoms indicating possible malaria infection.
Shigellosis (Bacterial Dysentery) outbreak in Bridgetown
Residents, particularly those living in crowded or unsanitary conditions, can be susceptible to Shigellosis infection. The causative agent is the Shigella bacterium, often transmitted through contaminated food, water, or person-to-person contact. Poor sanitation, unsafe water sources, and inadequate hygiene facilities contribute to disease transmission.
Interventional strategies include:
- Access to clean water and proper sanitation facilities can reduce environmental contamination.
- Education on handwashing and food safety can help interrupt disease transmission.
- Timely medical treatment can reduce the severity and duration of illness.
Tuberculosis, blunt the spread in Bridgetown
Identifying individuals at higher risk of TB, such as those living in crowded urban settings, is essential. Targeted TB screening and treatment programs can help control the disease. The agent is the Mycobacterium tuberculosis bacterium. Understanding drug-resistant TB strains and their prevalence informs treatment regimens. Overcrowded and poorly ventilated housing conditions can facilitate TB transmission. Improved housing and infection control measures in healthcare settings are crucial. Vaccination using BCG vaccine can offers some protection in children against severe TB such as TB meningitis and miliary TB. However, adolescents and adults are mainly impacted by pulmonary and latent TB against which the vaccine is ineffective requiring robust healthcare strategies to protect the populations.
Community engagement and education play a vital role in raising awareness about these diseases, promoting preventive measures, and ensuring access to healthcare services. Surveillance systems should be in place to detect outbreaks early, allowing for rapid response and containment.
Additionally, vaccination programs, if available, can help protect against certain diseases. Collaboration with international health organizations and NGOs can provide resources and expertise to support these efforts, especially in areas with a high poverty rate. By applying the principles of the epidemiologic triad to Malaria, Shigellosis, and Tuberculosis, Bridgetown can develop targeted interventions that address the specific host, agent, and environmental factors contributing to disease transmission, ultimately protecting the health of the community and controlling outbreaks effectively.
Disease Transmission
Diseases portraying an “iceberg effect” is a well-recognized concept in epidemiology. This effect suggests that for many diseases, only a small portion of cases are clinically evident and even a smaller number is diagnosed, while a larger portion remains subclinical or asymptomatic. This has several implications for disease transmission and epidemiology.
There are three major outcomes arising from this phenomenon:
a. Underreporting and disease surveillance
b. Herd immunity
c. Silent transmission.
Diseases with subclinical or asymptomatic cases can pose challenges for disease control and surveillance. These cases can contribute to ongoing transmission, making it important to implement comprehensive public health measures, including vaccination, early detection, and contact tracing, to prevent and control the spread of such diseases. The presence of a substantial number of subclinical cases can lead to underreporting of the disease. Since these cases do not seek medical attention or may not be recognized as part of the disease, they often go unreported. This can impact disease surveillance efforts, making it challenging to get an accurate picture of the disease’s prevalence and spread. Diseases with subclinical cases can silently spread in the community. People with asymptomatic infections may still be shedding the infectious agent and can unknowingly transmit the disease to others. This silent transmission can contribute to outbreaks and sustained transmission within populations. In some cases, underreporting and silent transmission may be beneficial as subclinical infections can contribute to the development of herd immunity. Herd immunity occurs when a sufficient proportion of the population becomes immune to a disease, either through vaccination or natural infection. In diseases with subclinical cases, a portion of the population may become immune without ever experiencing clinical symptoms.
It is important to note, promoting herd immunity through natural infection exposure, also known as “natural herd immunity,” is a contentious approach to disease control. There are several significant issues and concerns associated with this approach. Allowing a large proportion of the population to become infected naturally can lead to severe health consequences. For diseases like COVID-19, many individuals experience severe illness, hospitalization, and even death. Relying on natural infection for herd immunity could result in a high burden of disease and strain on healthcare systems. The long-term effects of the disease may be unknown which poses additional challenges for health protection. Disease severity can vary widely among individuals; some may have mild or asymptomatic cases, while others experience severe outcomes including death. Vulnerable populations, such as the elderly, immunocompromised individuals, and those with underlying health conditions, are at a higher risk of severe illness and death. Allowing a communicable disease to spread unchecked can lead to a surge in hospitalizations, overwhelming healthcare systems, and reducing their capacity to care for other health needs. The longer a virus circulates in a population, the more opportunities it has to mutate and develop new variants. Some variants may be more transmissible or resistant to immunity [example COVID-19, Omicron variant more transmissible than Delta variant] potentially undermining herd immunity efforts. Vaccines are readily available for some diseases and provide a safer way to achieve herd immunity. Deliberately allowing people to become infected for the sake of herd immunity raises ethical concerns as individual’s health and life are sacrificed for a collective goal. This can erode public confidence in public health measures and vaccination efforts, making long term health promotion and protection activities more challenging. Natural infection does not provide the level of targeted control that vaccination programs offer making it a high risk strategy with severe health, ethical and logistical challenges for planning health services.
Let’s us take a look at some examples to put things into perspectives.
Example 1.
Before the polio vaccine, many cases of polio were indeed subclinical or had mild symptoms. These individuals could still shed the poliovirus, contributing to its continued circulation.
Example 2.
Tuberculosis (TB) is known for its latent infection stage, where individuals can be infected with Mycobacterium tuberculosis without showing clinical symptoms. These latent TB infections can later become active, leading to the development of active TB disease.
Example 3.
In its early stages, rabies infection can be asymptomatic. Once clinical symptoms appear, the disease is almost always fatal. This makes early intervention critical in cases of potential exposure.
Example 4.
Some measles cases may be subclinical or have mild symptoms. Measles is highly contagious, and subclinical cases can still transmit the virus to others in the population.
Diseases with subclinical or asymptomatic cases can pose challenges for disease control and surveillance. These cases can contribute to ongoing transmission, making it important to implement comprehensive public health measures, including vaccination, early detection, and contact tracing, to prevent and control the spread of such diseases.
Modes of Disease Transmission
Modes of disease transmission are crucial concepts in epidemiology, helping to understand how infectious agents are spread from one host to another. These modes include contact, vehicle, and vector transmission. These can be further sub-divided into direct or indirect transmission, food/waterborne, etc, and horizontal or vertical transmission. Table 2.1 provides additional information on these modes of transmission. Disease transmission can occur due to single, multiple or continuous exposure to the infectious agent.
Understanding these modes of transmission is essential for designing effective public health interventions and preventive measures to control the spread of infectious diseases. Public health strategies often focus on interrupting these modes of transmission through vaccination, vector control, sanitation measures, and health education.
Table 2.1 Modes of Disease Transmission
Direct and Indirect Mode of Disease Transmission.
Understanding both direct and indirect modes of transmission is essential for public health efforts aimed at preventing and controlling infectious diseases. Public health interventions often focus on interrupting these transmission pathways through measures like vaccination, improved hygiene practices, vector control, and education to reduce the risk of transmission and protect populations from infectious disease outbreaks. Since these modes of transmission play a crucial role in understanding and controlling the spread of infectious agents. Let’s look at what they are and some examples:
Direct Transmission
Direct transmission occurs when an infected individual directly passes the infectious agent to a susceptible host through physical contact or close proximity.
Example 1
One common mode of direct transmission is through respiratory droplets. When an infected person talks, coughs, or sneezes, tiny respiratory droplets containing infectious agents are expelled into the air. These droplets can be inhaled by individuals in close proximity, typically within 3-6 feet. Diseases like COVID-19, influenza, and tuberculosis are primarily spread through respiratory droplets. In the case of COVID-19, for instance, close contact with an infected person who expels respiratory droplets containing the SARS-CoV-2 virus can result in transmission.
Example 2
Sexually transmitted infections (STIs) are diseases that are primarily transmitted through sexual activity. Examples include HIV/AIDS, syphilis, and gonorrhea. When an infected individual engages in sexual activity with a susceptible partner, the pathogens responsible for these diseases can be directly passed from one person to another through bodily fluids (e.g., blood, semen, vaginal fluids).
Example 3
Skin-to-skin contact or contact with mucous membranes can also facilitate direct transmission. For example, the human papillomavirus (HPV), which causes genital warts and is linked to cervical cancer, can be transmitted through direct skin-to-skin contact in genital areas. Herpes simplex virus (HSV), responsible for oral and genital herpes, can be transmitted through direct contact with affected skin or mucous membranes during oral, vaginal, or anal sex.
Indirect Transmission
The transfer of infectious agents from an infected individual to a susceptible host is facilitated by an intermediary such as contaminated objects, surfaces, or biological intermediaries.
Example 1
Fomites are inanimate objects [e.g. toys, cushions] or surfaces [e.g. countertops, doorknobs] that can become contaminated with infectious agents and serve as intermediaries for transmission. An individual with a respiratory infection who touches their mouth or nose and then touches a doorknob. Another person who touches the same doorknob and then touches their face can introduce the infectious agents into their body. Common cold viruses and gastrointestinal pathogens like norovirus often spread through fomite transmission.
Example 2
Vehicles, such as water, food, or blood products, can serve as vehicles for the indirect transmission of infectious agents. Waterborne diseases like cholera are often transmitted when individuals consume contaminated water. Foodborne illnesses, such as salmonellosis or E. coli infections, can occur when people ingest contaminated food products. Additionally, bloodborne pathogens like hepatitis B and hepatitis C can be transmitted through transfusion of contaminated blood or blood products.
Example 3
Indirect transmission can also involve vectors, which are living organisms (often arthropods) that carry infectious agents from one host to another. Mosquitoes can serve as vectors for diseases like malaria, dengue fever, and Zika virus. When a mosquito feeds on the blood of an infected person, it can acquire the pathogen. Subsequently, when the same mosquito feeds on another person, it can transmit the infectious agent, leading to disease transmission.
Horizontal and Vertical Transmission
Understanding both horizontal and vertical transmission is crucial for preventing and controlling the spread of infectious diseases. Evidence-based public health measures, such as vaccination, safe sexual practices, vector control, and maternal screening arise consequently to data gathered in these areas on disease transmission. Having knowledge and skills relating to these modes of transmission can help reduce the risk of disease transmission and outbreaks in various settings.
Horizontal Transmission
Horizontal transmission refers to the spread of infectious agents from one individual to another within the same generation. It can occur through various mechanisms and can involve single, multiple, or continuous exposures.
Table 2.2 Horizontal Mode of Disease Transmission.
Vertical Transmission
Vertical transmission occurs from an infected parent to their offspring, typically during pregnancy, childbirth, or breastfeeding. Simply, vertical transmission occurs from one generation to another.
Example 1
An HIV-positive mother can transmit the virus to her child during pregnancy, childbirth, or breastfeeding. This vertical transmission can result in the child being born with HIV or acquiring it through breast milk.
Example 2
A pregnant woman infected with the rubella virus can transmit the virus to her developing fetus. This can lead to congenital rubella syndrome, which can cause birth defects in the newborn.
Example 3
A pregnant woman with untreated syphilis can transmit the bacterium Treponema pallidum to her unborn child. This can result in congenital syphilis, leading to various health problems in the newborn.
Vertical transmission prevention strategies play key roles in pre-conception, pre-partum and post-partum health services delivery. Activities may take the form of screening, primary and secondary prevention to promote health of the mother and protect the health of the unborn child.
Summary
The natural history of disease is a critical concept in public health as it provides a framework for understanding the progression of illnesses within populations. It encompasses the stages of disease development, from the initial exposure to the ultimate outcome, whether it be recovery or death. Understanding the natural history of disease allows public health professionals to identify key points for intervention and implement strategies for prevention, early detection, and treatment. For instance, by recognizing the preclinical stage of a disease, interventions can be targeted to individuals at risk before symptoms appear, potentially mitigating the impact of the disease.
Moreover, understanding the mode of disease transmission is intricately linked to the natural history of disease. The mode of transmission dictates how a disease spreads within a population, whether through direct contact, vectors, vehicles, or other mechanisms. This knowledge is indispensable for designing effective control measures, whether it involves vaccination campaigns, vector control, quarantine, or public health education. In essence, the natural history of disease and the mode of transmission are intertwined aspects of epidemiology that guide public health efforts to prevent, manage, and mitigate the impact of diseases on communities and populations.
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Last updated: 2023- Sept- 12