Epidemiologic triad and chain of infection
- Concept
An infection needs an agent, a susceptible host and an environment that brings them together; breaking one link stops transmission.
What it is
The epidemiologic triad explains infectious disease as the interaction of an agent, a host and the environment that brings them into contact. The chain of infection follows the agent's path: reservoir, portal of exit, mode of transmission, portal of entry and susceptible host. Control measures work by breaking one or more links.
Structure
- Reservoir → portal of exit → mode of transmission → portal of entry → susceptible host.
- A carrier (infected without symptoms) can be a reservoir and a source of spread.
- Zoonoses have an animal reservoir; yellow fever and dengue need a vector.
Worked example
- Data (made up): in a boarding school of 240 students there is diarrhoea; 90 drank water from a tank and 150 only bottled water.
- Ill: 54 of the 90 who drank tank water (54 / 90 = 60 %) and 6 of the 150 on bottled water (6 / 150 = 4 %).
- Relative risk: 60 % / 4 % = 15; the tank is the main suspect.
- Reservoir: an infected person; portal of exit: their faeces, which reached the tank through a sewer leak.
- Mode of transmission: indirect, through a vehicle (water); portal of entry: the mouth.
- Host: all 240 students were susceptible; the 6 cases on bottled water suggest person-to-person spread.
- Control: closing and disinfecting the tank breaks the vehicle; handwashing and enteric isolation break the exit and contact.
- Reading: with two links broken, new cases should fall within one incubation period.
Each link of the chain and a measure that breaks it
| Link | Example | Measure that breaks it |
|---|---|---|
| Reservoir | Infected person, rabid dog | Treat the case, vaccinate animals |
| Portal of exit | Faeces, respiratory secretions | Hand hygiene, a mask on the sick person |
| Mode of transmission | Water, food, mosquito | Safe water, cooking, vector control |
| Portal of entry | Mouth, broken skin | Washing food, covering wounds |
| Susceptible host | Unvaccinated child | Vaccination, good nutrition |
Markers
- Agent
- Host
- Environment
- Reservoir
- Vector
In practice
- In an outbreak, laying out the chain points to the fastest control measure: removing the vehicle is almost always easier than treating everyone.
- In Colombia, vectors (Aedes, Anopheles, triatomine bugs) explain much of the infectious burden: environment is part of the triad.
Common pitfalls
- Calling the vehicle a reservoir: contaminated water carries the agent but is not where it normally lives.
- Forgetting the host: the same exposure makes some people ill and not others, depending on their immunity.
Test yourself
Think of the answer, then tap the question to see it.
What are the three elements of the epidemiologic triad?
The agent, the host and the environment that brings them into contact.
What are the links of the chain of infection, in order?
Reservoir, portal of exit, mode of transmission, portal of entry and susceptible host.
What is the difference between a reservoir and a vehicle?
In the reservoir the agent lives and multiplies; the vehicle (water, food, an object) only carries it from one place to another.
Which link does vaccination act on?
The susceptible host: it makes the person immune and leaves the agent nowhere to multiply.
Elsewhere in DurgiPedia
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Principles of Epidemiology | Lesson 1 - Section 10 (Centers for Disease Control and Prevention, 2012).
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Representative model, for educational purposes only.