Epidemiology and stages of a disease


Since the dawn of human history, our species has been plagued by disease. It wasn’t until the Renaissance, when the scientific method was first applied to medicine, that we began to make real progress in the fight against diseases. Epidemiology is concerned with the distribution and determinants of health and disease among human populations. It is an interdisciplinary field and draws on scientific and social knowledge from many disciplines, including medicine, social science, statistics, and biology. The insights and principles of epidemiology guide various public health policies and resource allocation, which in turn improves population health.

Epidemiologists study the prevalence of disease in different populations and how the risk of disease changes with age and other factors. Statistical and mathematical models are used to understand these relationships. 

Principles of Epidemiology 

To understand how infectious diseases spread in a population, we need a clear understanding of the principles of epidemiology and how they can be applied to the control of these diseases. 

Disease information is collected from a population to obtain epidemiological data. Disease-reporting surveillance networks, clinical records, and patient interviews provide data used to identify common factors in illnesses. This data is then analysed and interpreted. The insights drawn from this data are used to contribute to the principles of epidemiology. 

As pathogens must grow and multiply in the host to cause disease, epidemiologists track the natural histories of pathogens. Pathogens that are well adapted to their host live in balance with them and cause them minimal harm. These pathogens might cause chronic infections (long term) in the host. 

Principles of epidemiology

Image Source: onlinedegrees.unr.edu

Sometimes, an individual host has not developed resistance to a newly emerging pathogen. Such pathogens can lead to acute infections, which are often rapid and dramatic. To formulate effective strategies, it is necessary to know both the population dynamics and clinical problems associated with a disease. 

Acute disease chronic diseases

Source: www.researchgate.net

Vocabulary of Epidemiology 

The word “epidemiology” comes from the Greek words epidemia, meaning “public health,” and logos, meaning “study of.”  The science of epidemiology is a language of its own. It requires you to understand certain basic terminology.

Epidemiology meaning

Source: libguides.acom.edu

Here are a few of those terms:-


When a disease infects a large number of individuals in a population at once, it is said to be an epidemic. 


A pandemic is a global and extensively spread epidemic. E.g.- COVID-19. 


Conversely, the endemic disease is continually present in the population, often to a lower degree.


Sporadic cases of a particular disease appear one at a time in different geographical regions.


Whereas, a disease outbreak is the sudden appearance of several cases in an area previously exhibiting only sporadic cases.

Principles of epidemiology

Image Source: www.mayo.edu

Subclinical infections are exhibited by diseased individuals who show mild to no symptoms. Such individuals are carriers of the disease. Two terms frequently used while dealing with population and disease are prevalence and incidence. They are indicators of public health of a particular group such as a city, state, or country. The prevalence of a particular disease is the total number of new and existing disease cases in a population in a given period. Whereas the incidence of a particular disease is the number of new cases in a population in a given period. Therefore, incidence provides a record of new cases of a disease, while prevalence indicates the total disease burden in a population.

World health day

Disease Progression (stages of a disease)

Disease progression is the series of changes in the body over time in response to a disease. It can be caused by many different things, such as the disease itself, the body’s reaction to the disease, and the treatment of the disease. A typical acute infection progresses in four stages:

1. Infection: The pathogen invades, colonizes, and grows in the host.

2. Incubation period: The incubation period is the time from initial exposure to the pathogen until the first clinical symptoms of the disease. It varies from disease to disease.

  • For example, Chickenpox is a highly infectious disease that has an incubation period of about two weeks, while the incubation period for the common cold can be as short as two days. At the end of incubation, the first symptoms, such as headache and a feeling of illness, appear.

 3. Acute period: This is the period when the disease is at its peak, with symptoms such as fever and chills.

Principles of epidemiology

Image Source: www.istockphoto.com

4. Decline period: The disease symptoms decline, fever subsides usually followed by a period of intense sweating and a feeling of well-being. It is said to occur by crisis if the decline is rapid (within 1 day) or by lysis when it stretches over several days.

5. Convalescent period: The individual regains strength and returns to normal.

While there is no way to prevent disease progression in all cases, there are ways to slow down disease progression and help the body fight off the disease. The stages of a disease are the most important parameters for the principles of epidemiology. 

Also read: Is Vaccination a key to Immunization? (mybiologydictionary.com)

Disease Reservoirs 

Reservoirs are sources of diseases in which infectious agents remain viable. They could potentially infect healthy individuals. A reservoir may be animate or inanimate. 

Inanimate reservoirs typically belong to saprophytic pathogens. They only infect humans by chance and cause disease. For example, tetanus is caused in animals by Clostridium tetani, which normally inhabits soil, and infection of a host is not essential for the bacterium to continue its existence. Thus, it causes infection in animals accidentally and can survive in nature in the absence of hosts. On the other hand, for many pathogens, living organisms are the only reservoirs. In such organisms, the reservoir host is essential for the continuation of the life cycle. Various viral and bacterial respiratory pathogens need human hosts to continue their life cycle. 

Clostridium tetani Tetanus

Image Source: www.sanofi.com


When a disease is passed from an animal to a human, it is referred to as zoonosis. The carrier of disease can range from an infected animal to a person who has been in contact with the disease. 

Some of the most common zoonotic diseases include rabies, hepatitis, and Lyme disease. Bubonic Plague, which is caused by Yersinia bacteria, is transmitted from animals to humans through the bite of a flea. These diseases can be transmitted to humans through a variety of routes, including bites, scratches, and contact with bodily fluids.

The most common zoonoses are caused by bacteria and viruses, but some zoonoses are caused by parasites, such as worms, or by other types of pathogens, such as prions. Elimination of the human form of zoonotic disease can be only achieved through the eradication of the disease in animal reservoirs. 


Image Source: www.cdc.gov


The carrier of a disease is a person who is infected with the pathogen that causes the disease. The disease can then be transmitted to other people through the carrier’s infected bodily fluids or tissues. In some cases, the disease can be transmitted through the air i.e., in the case of airborne diseases, such as the common cold or the flu. 

In other cases, the disease can only be transmitted through direct contact with the infected carrier, such as with the Ebola virus. An infamous carrier responsible for a typhoid outbreak in New York during the early 1900s was Mary Mallon, also known as, ‘Typhoid Mary’.

Typhoid Mary

Image Source: www.gavi.org

Transmission of infectious disease

Epidemiologists follow the transmission of a disease by correlating geographic, climatic, social, and demographic data with disease incidence. 

Transmission refers to how infectious disease is spread from one person to another. Infectious diseases can be transmitted in a variety of ways and through different vectors, including the air, the water, and the ground. This pathogen transmission can be direct or indirect. 

Direct Host-to-Host Transmission

Infectious agents may be transmitted through direct contact, such as through physical contact with another individual or through contact with a bodily fluid or other material that is infected, such as through contact with faeces or blood. This type of transmission is known as direct host-to-host transmission. 

Upper respiratory infections such as the common cold and influenza are most often transmitted host to host by droplets resulting from sneezing or coughing. 

Principles of epidemiology

Image Source: www.vectorstock.com

Indirect Host-to-Host Transmission

Pathogens may also be transmitted through indirect contact, such as through contact with objects that have been contaminated with infectious agents. This indirect transmission of the disease is a common occurrence in healthcare settings where there is a constant exchange of material between patients, healthcare workers, and equipment. These inanimate objects are collectively called fomites. Indirect transmission can also occur through vectors, such as through the transfer of infectious agents from one animal to a human through the bite of an infected insect or animal, such as a mosquito or tick.

The most common vectors in humans are mosquitoes, which can carry viruses such as the West Nile virus and Dengue fever. Other vectors in humans include ticks, lice, and fleas.

One of the most important scientific discoveries of the 20th century was the role of the “vector” in the transmission of disease.


Image Source: www.vectorstock.com


Epidemics are a serious threat to human health and life. The Black Death, the Spanish Flu, HIV, and many other epidemics have killed millions of people and infected countless others. Epidemiologists track the spread of disease, look for patterns, and design control strategies to minimize the spread of the disease, these ultimately govern the principles of epidemiology.

Major epidemics are usually classified as either common-source epidemics or host-to-host epidemics.


Image Source: physicsworld.com

Common-source epidemics

Common-source epidemics are those where an infectious agent is transmitted primarily through a common vehicle (e.g. food, water, air). As such, common-source epidemics tend to be more easily contained and limited in geographical extent. 2009 H1N1 pandemic, previously referred to as the “swine flu”, is an example of a common-source epidemic.  

Host-to-host epidemics

In contrast, in host-to-host epidemics, the infectious agent is transmitted from one person to another through direct contact (e.g. through scratches). Host-to-host epidemics can spread more rapidly and widely. 

Coevolution of Host and Pathogen

It has been observed that the evolution of infectious diseases has occurred in close association with the evolution of the organisms that host them.

Coevolution is a process by which two or more species evolve in a way that gives an evolutionary advantage to both. 

The host plays a major role in the coevolution of a pathogen, serving as a selective pressure that shapes the way the pathogen evolves. The relationship between a pathogen and the host is mutually beneficial, but the pathogen’s ability to survive and replicate within the host is limited by the host’s ability to defend itself and respond to the pathogen. 

Herd Immunity

In populations where a high proportion of people are immune, the pathogen is unable to spread easily from person to person, and the disease is usually rare; this resistance to infection is called herd immunity

There are many ways to increase the proportion of people who are immune. For instance, vaccination, improving sanitation and access to clean water, and educating the public about immunization. However, people who are unable to be vaccinated for medical reasons, such as infants, the elderly, and those with allergies, can still protect those around them by avoiding exposure to the disease. 

To put it all together 

For as long as humans have lived on this planet, we have been exposed to a variety of microorganisms. These microbes have had a profound impact on our health, our behaviour, and our evolution as a species which ultimately affects the principles of epidemiology. 

COVID-19 vaccines

Image Source: www.hopkinsmedicine.org

In some cases, exposure to certain microbes has been beneficial, while in others it has been harmful. The organisms that cause disease have evolved alongside us, adapting to our immune systems, our environments, and our behaviours. Epidemiology has had a major role in the practice of disease investigation and improved public health. The principles of epidemiology have helped us to keep our communities healthy and safe. The idea is to promote an environment where people can live long healthy lives

Intrigued by the science of epidemiology? Check out our blog for more amusing scientific enlightenment. 

Team MBD

Read more- Principles of Epidemiology | Lesson 1 – Section 1 (cdc.gov)

1 Response

  1. Anoopkotnala says:

    Very informative and knowledgeable.

Leave a Reply

Your email address will not be published. Required fields are marked *