3 Infectious Diseases

Infectious disease

An infectious disease is an illness caused by pathogens (e.g., bacteria, virus) that enter the body, multiply, and cause infection.

Communicable disease

A type of infectious disease that can spread from one person or animal to another.

Pathogen

A pathogen is an infectious agent that can cause disease or illness. Pathogens include viruses, bacteria, protozoans, prions, and fungi.

Asymptomatic infection

An infection in a person who does not have any signs or symptoms of the disease. Sometimes also called a ‘subclinical infection’.

Reservoir

Any person, animal, plant, soil or substance in which an infectious agent normally lives and multiplies. A reservoir is sometimes called a ‘source’ when the infectious agent in the reservoir is the cause of a person getting sick.

Host

Any large organism in which a smaller organism lives. In field epidemiology, hosts are usually humans or animals. A ‘susceptible host’ is a host that is able to be infected by an organism. For example, a person who has not previously been infected with the agent or who is not vaccinated against infection may be susceptible. Other factors that may make a person susceptible to infection are chronic health conditions, pregnancy or genetics.

Chain of infection

A process that begins when an infectious agent leaves its reservoir and travels by a route of transmission, then enters and infects a susceptible host.

This is shown in the pictures below:

A sequence of icons and labels explaining the transmission of infectious diseases. Starting with a virus icon labeled ‘Pathogen,’ followed by icons of a bird, two people, and a pond representing ‘Reservoir/Source.’ The next icons depict a kiss, a cough, a chicken drumstick and a mosquito, indicating the ‘Route of Transmission.’ The final part of the chain shows two human figures side by side, labeled as ‘Susceptible Host.’

Figure 2: Chain of infection for the spread of a pathogen to a susceptible host

 

A sequence of four icons. The first icon represents the pathogen, Vibrio cholerae. The second icons depict a lake and a human small intestine, representing the reservoirs. The third icon shows routes of transmission, symbolized by a hand holding a glass of water, indicating ingestion of contaminated water or food, and a toilet icon representing direct transmission from poor sanitation and hygiene. The final icon represents the hosts, humans, with a group of stylized figures.

Figure 3: Example chain of infection for the spread of cholera

Route of Transmission

An infectious agent may travel from its reservoir to a susceptible host in different ways. Common routes of transmission are:

Direct transmission

The immediate transfer of an infectious agent from a reservoir to a susceptible host.

Direct contact

Occurs through skin-to-skin contact, kissing, and sexual intercourse. Direct contact also refers to contact with soil or waste products (e.g. faeces) that are contaminated with an infectious agent.

Examples:

Gonorrhoea is spread from person to person by sexual intercourse.

Hookworm is spread by direct contact with contaminated soil.

Droplet transmission

When an infected person sneezes, coughs, or talks, they make droplets containing the infectious agent. The droplets spray over a short distance (1–2 metres) and any susceptible host who breathes in the droplets can become infected.

Example:
Pertussis, influenza and invasive meningococcal disease are examples of diseases transmitted from an infectious patient to a susceptible host by droplet spread.

Indirect transmission

The transfer of an infectious agent from a reservoir to a susceptible host by particles that are suspended in the air (airborne), inanimate objects (vehicle-borne), or animals, insects or parasites (vector-borne).

Airborne transmission

Occurs when infectious agents are carried by tiny, lightweight dust or droplets suspended in air. Whilst heavier droplets fall to the ground quickly, with airborne transmission, the infectious agent may remain in the air for long periods and can be blown over long distances.

Example:

Measles is an airborne pathogen.

Vehicle-borne transmission

Vehicle-borne transmission is the indirect transmission of infectious agents through vehicles such as water, food, biological products such (e.g. blood), and fomites (inanimate objects such as bedding or surgical scalpels).

Example:

People infected with Vibrio cholerae may defecate near a drinking water source. The drinking water can become contaminated with the bacterium and it spreads to other people who drink the contaminated water.

Vector-borne transmission

Vectors are animals, insects or parasites that carry infectious agents. Common vectors are mosquitoes, fleas, flies and ticks. Vectors can transmit the infectious agent from one host to another through bites, stings or touch. For some infectious agents, the agent must grow or change in the vector before it can be transmitted.

Example:

Flies can carry Shigella on their feet and fleas carry Yersinia pestis, the infectious agent that causes plague, in their gut. Mosquitoes carry the parasite that causes malaria, but the parasite must mature in the mosquito before it can be transmitted to humans.

Epidemiologic Triangle

The diagram used to describe how infectious agents cause disease includes three parts: 1) an infectious agent, 2) a susceptible host, and 3) an environment that brings the host and agent together, so that disease occurs.

Diseases or health events occur because of interactions between the host, the environment, and the infectious agent. This is called the epidemiologic triangle. Understanding how the three parts interact using the epidemiologic triangle is important in helping us think about how diseases are being spread. Public health action aims to stop the interaction between two or more parts of the triangle to prevent disease. Sometimes there are multiple public health actions that we can take, and we need to consider the benefits of each one to decide which to use.

A triangle showing the dynamic interactions among the three critical factors in disease outbreak and transmission: ‘Host’ at the top vertex, ‘Agent’ at the bottom left, and ‘Environment’ at the bottom right. Each vertex is interconnected with bidirectional dotted lines, symbolizing the complex and continuous relationships that influence the patterns of disease occurrence.
Figure 4: Epidemiologic triangle

In field epidemiology, when we make decisions about which public health actions to take, we need to consider the following:

Table 3: Using the epidemiologic triangle when considering the public health actions to take

Infectious agent
  • What is the cause or likely cause of the illness?
  • How does it spread?
  • How long does it survive in the environment?
  • What can be done to control or eliminate the agent?

Susceptible host

 
  • Who is most affected by the disease?
  • Are they elderly or children?
  • Do they have another health condition or weakened immune system?
  • Are they vaccinated?
  • Why are they more likely to get infected?
  • What can we do to reduce the susceptibility of other potential hosts (i.e. people not yet infected)?

Environment

 
  • What environmental factors are contributing to the spread of the disease?
  • Is the disease occurring in the city where people live close to each other?
  • Is the disease occurring in a rural area where people may live close to animals?
  • Do the people getting sick live near water?
  • Is the water source potentially contaminated?
  • How can the environment be altered to prevent transmission?

 

Example of the epidemiologic triangle and how we can use this to consider public health actions to prevent cholera:

At the centre, there is a large triangle labelled ‘Cholera.’ Each corner of the triangle points to a specific element of the cycle: Host: Humans are represented here. Measures to protect humans include washing hands, making water safe, and cooking foods. Agent: The pathogen responsible for Cholera is Vibrio cholerae, transmitted through contaminated water/food and the vehicle-borne route of transmission. Environment: Strategies include having available latrines and locating them away from drinking water sources. Dotted lines and bidirectional arrows connect these elements to show their interrelation in transmitting Cholera.
Figure 5: Epidemiologic triangle for cholera

Stages of disease

By knowing the stages of disease, we can get an idea of the time when a person became sick (to find the source of their infection) and the time when they were infectious (to work out who they may have spread the infection to).

The length of each stage is different for each disease and is estimated with a range because the stages can differ for each person depending on their physical characteristics or the amount of the infectious agent they were exposed to.

Incubation period

The incubation period is the time between being exposed to a pathogen and when symptoms of the disease begin. During this time, the infectious agent is establishing itself in the host’s body, but the host does not know they have been infected. By knowing the incubation period, we can investigate when the host was likely exposed and where the host may have got their infection from.

Example:

The incubation period of COVID-19 is typically 2–14 days. The average incubation period, however, is 5 days, with some variants having shorter incubation periods.

Latent period

The latent period is the time between a host being exposed to a pathogen and when they become infectious. This is sometimes called the pre-infectious period. The pathogen is inside the host, but the host cannot spread the pathogen to other hosts during this time.

Clinical disease

Clinical disease is the time when the host has signs and symptoms of the disease. It begins when symptoms start (onset) and finishes when the symptoms end, either because the host has recovered or died from the disease.

Infectious period

The infectious period is the time during which an infected host is able to pass the infectious agent onto another host.

Often, a host is infectious once they have symptoms, such as with pertussis (whooping cough). Sometimes, however, a host can be infectious without symptoms or before the symptoms start. An example is COVID-19, where a person can be infectious up to 48 hours before symptom onset.

Rapid contact tracing is particularly important in outbreaks of diseases where people are infectious before symptom onset, as they are likely to have been living their life normally at work, school, socially, and home – and therefore exposing many people – before realising that they were infectious.

In the case of measles, the person is infectious up to four days before a rash appears and one day before onset of any symptoms (as indicated in the area shaded BRIGHT GREEN in the image below).

Example of the stages of disease below, using measles as an example:

Graph of the stages of a disease. Latent Period (1 June to 10 June): the time from when a host is infected to when they become infectious. Incubation Period (1 June to 11 June): the period from exposure to a pathogen to when symptoms first appear. Infectious Period (10 June to 17 June): when an infected person can spread the disease to others. Onset of first symptoms occurs on 11 June (indicated by a bright green rectangle). Clinical Disease (from 11 June to resolution of symptoms): symptoms are present, including a rash on 15 June. At the end of the Clinical Disease period (after 17 June), symptoms are resolved.
Figure 6: Stages of an infectious disease

 

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A Reference Guide for Field Epidemiologists Copyright © 2025 by University of Newcastle is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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