Draw and Label the Chain of Infection

All 11 boxes on the diagram

Show the list

The six links 6

Infectious agent The germ that can cause an infection. Bacteria, viruses, fungi and parasites are agents.
Reservoir Where the agent normally lives, grows and multiplies. People, animals, soil and water can be reservoirs.
Portal of exit The path by which the agent leaves its reservoir. The nose and mouth are one.
Mode of transmission The way the agent travels to a new host. Five ways are named below the ring.
Portal of entry The path by which the agent enters a new host. Often it matches the portal of exit.
Susceptible host A person who can be infected by the agent. It is the last link.

Modes of transmission 5

Direct contact The agent passes by touch, from one person to another.
Droplet spread Spray from a cough or a sneeze. The drops stay in the air only a short time.
Airborne Very small particles carry the agent. They stay in the air for a long time.
Vehicle-borne Food, water or an object carries the agent.
Vector-borne A living carrier, such as a mosquito, carries the agent.

How this list was compiled: Every name checked on 2026-10-02 and kept only where two of these use it. (1) United States Centers for Disease Control and Prevention, Principles of Epidemiology in Public Health Practice, third edition, Lesson 1, Section 10, Chain of Infection, a public-domain text read in the agency's archive (the agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate portal of entry to infect a susceptible host; the reservoir is the habitat in which the agent normally lives, grows and multiplies, and reservoirs include humans, animals and the environment; one classification of the modes: direct contact and droplet spread, which are direct, and airborne, vehicleborne and vectorborne, which are indirect; vehicles include food, water and objects; vectors such as mosquitoes, fleas and ticks; the final link is a susceptible host). (2) Nursing Assistant, section 4.2, Chain of Infection, an openly licensed textbook (six links: infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host; the agent may be a bacterium, a virus, a fungus or a parasite; a reservoir can be a person, an animal or the environment; the agent usually leaves through a person's nose or mouth; the portal of entry is the reverse of the portal of exit; modes include contact, droplet or airborne transmission). (3) Wikipedia, Infection and Pathogen transmission (the chain of infection includes the infectious agent, a reservoir, exit, transmission and entry into a susceptible new host; direct contact by touching; droplet transmission, particles that stay in the air for a short period; airborne transmission, very small particles that stay in the air for long periods; vehicle transmission by food, water or soil; a vector such as a mosquito). Licence: own work

The drawing: Drawn for this site on 2026-10-02 from plain geometry. Own original chart; nothing traced or copied. Licence: own work

How it works

To draw and label the chain of infection, name its links in order: infectious agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host. Five more boxes name the ways an agent travels.

Read the full guide

Go around the ring

Start at the top with the germ, the infectious agent. The next picture is a person carrying it, the reservoir. The agent leaves by a portal of exit, travels by a mode of transmission and enters a new person by a portal of entry. That person is the susceptible host.

Name the ways it travels

A bracket under the ring leads to five pictures. Two people touching is direct contact and falling drops are droplet spread. Specks in moving air are airborne, a glass and an apple are vehicle-borne, and a mosquito is vector-borne.

Draw it yourself

Print blank, under More options, gives the chart with empty boxes. To draw your own, put six circles in a ring, join them with arrows and write one link by each.

Books sort the modes of transmission in different ways. Some list only contact, droplet and airborne.

Questions and answers

What are the six links in the chain of infection?

Infectious agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host, in that order.

What is a reservoir in the chain of infection?

The place where an infectious agent normally lives, grows and multiplies. A reservoir can be a person, an animal or part of the environment, such as soil or water.

What is the difference between a portal of exit and a portal of entry?

The portal of exit is the path by which the agent leaves its reservoir. The portal of entry is the path by which it enters a new host. Often the two are the same kind of path, such as the nose and mouth.

What are the modes of transmission?

One common list has five. Direct contact and droplet spread are direct. Airborne, vehicle-borne and vector-borne transmission are indirect: the agent rides on particles in the air, on food, water or objects, or on a living carrier such as a mosquito.