[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Epidemiology basics](https://durgical.com/en/epidemiology/areas/basics/) › Epidemiologic triad and chain of infection

# 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.

Updated Nov 5, 2026

## Key points

- Triad: agent (the microorganism), host (the person) and environment (what makes contact possible).
- Reservoir: where the agent normally lives and multiplies (people, animals, soil or water); it is not the same as a vehicle.
- Modes of transmission: direct (contact, droplets) and indirect (airborne, vehicles such as water or food, and vectors).
- Portals of exit and entry: respiratory tract, gut, skin, mucous membranes or blood.
- The host is susceptible according to immunity, genetics and general defences; vaccination lowers susceptibility.

On this page

- The basics Key points
- What it is
- Structure
- Worked example
- Each link of the chain and a measure that breaks it
- Markers
- In practice
- Common pitfalls
- Memory aid
- Elsewhere in DurgiPedia

- Study Test yourself

- Connections Connections

## 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.

## Memory aid

Lives, Leaves, Travels, Enters, Infects: five verbs for five links, and breaking any one is enough.

## 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.

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## Elsewhere in DurgiPedia

- [Herd immunity and outbreak control](https://durgical.com/en/immunology/herd-immunity-and-outbreak-control/)

## Connections

## Related 5

- [Natural history of disease](https://durgical.com/en/epidemiology/natural-history-of-disease/)
- [Outbreak investigation](https://durgical.com/en/epidemiology/outbreak-investigation/)
- [Attack rate](https://durgical.com/en/epidemiology/attack-rate/)
- [Basic reproduction number (R0) and herd immunity](https://durgical.com/en/epidemiology/basic-reproduction-number-and-herd-immunity/)
- [Levels of prevention](https://durgical.com/en/epidemiology/levels-of-prevention/)

## Related conditions 3

- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Cholera](https://durgical.com/en/conditions/cholera/)
- [Dengue](https://durgical.com/en/conditions/dengue/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

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](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson1/section10.html) (Centers for Disease Control and Prevention, 2012).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/epidemiologic-triad-and-chain-of-infection/