A nursing instructor asks a student to list the six links of the chain of infection in order. Which list is correct?
Choose an answer, then check it.
Nursing & Allied Health Foundations · Foundations
Chain of Infection
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In 30 seconds
The chain of infection is a six-link model, taught from CDC materials, that explains how an infection spreads: infectious agent The germ that can cause disease: a virus, bacterium, fungus, or parasite. Full entry →, reservoir The habitat where an infectious agent normally lives, grows, and multiplies, such as a person, an animal, a surface, or water. Full entry →, portal of exit The path by which a germ leaves its reservoir or host, such as the respiratory tract during a cough. Full entry →, mode of transmission The way a germ travels from its reservoir to a new host, such as by touch, droplets, or air. Full entry →, portal of entry The route by which a germ enters a new host, such as the nose, mouth, broken skin, or bloodstream. Full entry →, and susceptible host A person who can catch an infection because they have no protection against that germ. Full entry →. All six links must be present, in order, for one person to pass a germ to another. Break any single link — wash your hands, put on gloves, clean a surface, vaccinate, isolate — and the spread stops. Every infection-prevention habit is a way of snapping the chain.
Why this matters
Nurses and other health workers meet germs every shift, but an infection is not bad luck — it is a sequence with six conditions that all have to line up. The chain turns that sequence into a checklist you can act on: name the link you can break, and you can stop a patient, a coworker, or a whole unit from getting sick. It explains why one patient's cough can become another patient's pneumonia, why a shared blood-pressure cuff deserves a wipe-down, and why the same small routines — hand hygiene, gloves, cleaning, vaccines — keep appearing in every safety conversation. Master the chain, and every precaution you learn later makes sense as the same move: break a link.
The college version
Six conditions that must line up
An infection is not a single event; it is a process with steps. The CDC's epidemiology teaching materials describe the chain of infection as the sequence in which an infectious agent leaves its reservoir through a portal of exit, is carried by a mode of transmission, and enters through a portal of entry to infect a susceptible host. Six links, in order, and every one must be present. The model matters because it turns “germs are everywhere” — true but not useful — into a list of specific conditions a person can check and interrupt. It is also why infection prevention feels repetitive: every precaution exists to break one of these six links.
The six links, one by one
Each link answers one question about an infection. Infectious agent — the germ itself: a virus, bacterium, fungus, or parasite that can cause disease. Example: the influenza virus that arrives in a waiting room. Reservoir — the place where the germ normally lives, grows, and multiplies. Example: the respiratory tract of the person carrying the flu. Portal of exit — the path by which the germ leaves its reservoir. Example: the nose and mouth when that person coughs. Mode of transmission — the journey the germ takes to a new host. Example: droplets from the cough landing on a nearby tray table, then carried onward on hands. Portal of entry — the route by which the germ enters the next person. Example: a visitor's nose, or a nick in the skin. Susceptible host — a person with no protection against the germ. Example: a visitor who has never had flu and did not get the vaccine. Ask those six questions about any infection — what is the germ, where does it live, how does it get out, how does it travel, how does it get in, who can catch it — and you have drawn its chain.
Reservoirs: where germs live
A reservoir is the habitat where a germ normally lives, grows, and multiplies. Health workers meet four broad kinds every day. People: a patient with symptoms, but also someone colonized — carrying the germ with no symptoms and able to pass it on. Animals: rabies lives in bats and raccoons, and many animal diseases can reach humans, which is why animal bites get prompt attention. Surfaces: bed rails, doorknobs, countertops, and medical equipment can hold germs and hand them to whoever touches next. Water: sinks, faucet aerators, and cooling towers can harbor organisms such as Legionella, the cause of a serious pneumonia. One distinction is worth keeping straight: the reservoir is where the germ normally lives; the source is where it came from in a particular case. The botulism germ lives in soil, but the source of most botulism cases is improperly canned food.
Modes of transmission
Germs do not travel by themselves; something carries them. Three modes dominate healthcare thinking. Contact: germs move by touch — skin to skin, or hand to surface to hand. Droplet: the larger spray from a cough, sneeze, or conversation travels only a short distance, roughly a few feet, before falling to the ground. Airborne: tiny particles stay suspended in the air and can travel much farther on air currents. The three modes matter because each demands different protection — which is exactly what the sibling lessons on standard precautions and transmission-based precautions cover in detail.
Breaking the chain
Every link has a counter, and the counters are daily habits. Hand hygiene breaks transmission: soap and water or hand sanitizer removes germs from hands before they move to the next patient. Personal protective equipment — gloves, gowns, masks — blocks the portal of entry for the person wearing it. Cleaning and disinfection attack the reservoir, removing germs from surfaces so there is nothing left to pick up. Vaccines break the susceptible-host link by giving the immune system a head start, so a vaccinated person is no longer an easy target. Isolation breaks the chain at the exit and the journey: keeping an infected person apart keeps the germ from reaching anyone else. The power of the model is that one break is enough — you do not need to defeat all six links, just one, at the right moment.
The honest framing
The chain is a checklist, not a magic diagram. Walk any real infection through the six questions and you will find the places to intervene — that is the point of the model. But real life is messier than the drawing: some germs have several reservoirs and several routes, some people spread germs while feeling perfectly fine, and some germs survive on surfaces for days. The chain will not predict exactly who gets sick. What it does is keep the whole sequence in front of you, so the right question — which link can I break right now? — is always available. The model is simple; the work of breaking links is not.
Eli explains
The same idea, in plain words
Explain it like I’m 10
The chain of infection is a way of seeing how a germ moves from one person to another. Picture six boxes in a row. Box one holds the germ. Box two is where the germ lives. Box three is how the germ gets out of its home. Box four is the journey — how it travels. Box five is the door it comes in through. Box six is the person who catches it. If every box works in order, someone gets sick. If even one box fails — the germ cannot get out, the journey is blocked, the door is shut, or the person is protected — the infection stops right there. That is why health workers repeat the same small actions every day: each one breaks a box.
Picture it like this
Think of the chain as a relay race where the baton is a germ. Runner one is the germ itself; runner two is the place it lives; the handoff is the way it leaves; the track is the route it travels; the receiving hand is the door it enters; and the final runner is the person who ends up holding the baton. If any runner drops the baton or fumbles the handoff, the race stops — and nobody finishes sick.
Where the picture stops working
A relay race is neat and linear, but real infections are messier. One germ can have several reservoirs, several routes, and several ways in, and a person can be exposed many times before any infection takes hold. The chain is a thinking tool, not a law of nature: it shows where to interrupt the process, but it cannot tell you exactly who will get sick.
Worked example
Consider a Tuesday on a medical unit. A patient has norovirus, a stomach bug that spreads easily: the infectious agent is the virus itself, and the reservoir is the patient's digestive tract. The virus leaves the body through stool — the portal of exit. A housekeeper's gloved hand touches the bathroom door handle and carries the virus to the sink faucet, where the nurse's bare hand lands a moment later — contact transmission. The nurse rubs her eye, and the virus enters through the mucous membrane — the portal of entry. She is a susceptible host because she has never had norovirus and has no immunity to it. Now replay the scene with one change: the nurse uses soap and water after touching the handle. The virus never reaches the faucet, the chain stops at link four, and nobody gets sick. One break, no infection.
Key takeaway
An infection spreads only when all six links of the chain line up; break any one — with hand hygiene, PPE, cleaning, vaccines, or isolation — and the spread stops.
Quick check
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
A patient with influenza coughs while a visitor stands two feet away. Droplets from the cough reach the visitor's nose, and three days later the visitor is sick. Which link of the chain is the visitor's nose?
Choose an answer, then check it.
A nurse is teaching a family member about stopping a germ that spreads by touch. The family member asks: “When we wash our hands before and after visiting, which link of the chain are we attacking?” Which answer is correct?
Choose an answer, then check it.
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related
You’ll learn to
- Define the chain of infection as the six conditions that must line up for an infection to spread, in the CDC's framing.
- Name the six links — infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host — and give an original example of each.
- Explain what a reservoir is and identify the main kinds: people, animals, surfaces, and water.
- Distinguish the three main transmission modes — contact, droplet, and airborne — by how far and how the germ travels.
- Apply the model by identifying which link a given prevention measure — hand hygiene, PPE, cleaning, vaccines, or isolation — breaks.
- Explain why breaking a single link is enough to stop an infection, and use the chain as a checklist for everyday practice.
Common mistakes
The source and the reservoir are the same thing.
The reservoir is where the germ normally lives and multiplies; the source is where it came from in a particular case. The botulism germ's reservoir is soil, but the source of a typical case is improperly canned food.
Droplet and airborne spread are the same because both come from coughing.
Droplets are larger and fall within a few feet, while airborne particles stay suspended and travel much farther — which is why the two call for different protection, covered in the transmission-based precautions lesson.
Only people who look sick spread germs.
People can be colonized or carry a germ with no symptoms and still pass it on, which is exactly why hand hygiene happens with every patient, not just the ones who look ill.
To stop an infection you have to break every link.
One solid break is enough. If the germ cannot reach a new host through even a single link — say, a vaccine made the host immune — the chain is broken, even though the other five links are intact.
Easily confused
Reservoir vs. Source
The reservoir is where the germ normally lives and multiplies; the source is where the germ came from in one specific case. A germ's reservoir can be soil while its source is a jar of improperly canned food.
Contact transmission vs. Droplet transmission
Contact moves germs by touch, direct or through surfaces and hands; droplet spread carries them a short distance through the air when someone coughs, sneezes, or talks. Both are common, but they demand different protections.
Susceptible host vs. Infected host
A susceptible host is anyone with no protection against a germ; once the germ takes hold, that person becomes an infected host — and often a new reservoir — so the chain can continue to the next person.
Key vocabulary
- infectious agent
- The germ that can cause disease: a virus, bacterium, fungus, or parasite.
- reservoir
- The habitat where an infectious agent normally lives, grows, and multiplies, such as a person, an animal, a surface, or water.
- portal of exit
- The path by which a germ leaves its reservoir or host, such as the respiratory tract during a cough.
- mode of transmission
- The way a germ travels from its reservoir to a new host, such as by touch, droplets, or air.
- portal of entry
- The route by which a germ enters a new host, such as the nose, mouth, broken skin, or bloodstream.
- susceptible host
- A person who can catch an infection because they have no protection against that germ.
- droplet spread
- Transmission by the larger spray of respiratory droplets that travels only a short distance, about a few feet, before falling.
- airborne transmission
- Transmission by tiny particles that stay suspended in the air and can travel farther on air currents.
Sources & references
- How Infections Spread — U.S. Centers for Disease Control and Prevention (CDC), National Center for Emerging and Zoonotic Infectious Diseases
- Principles of Epidemiology in Public Health Practice, Third Edition — Lesson 1, Section 10: Chain of Infection — U.S. Centers for Disease Control and Prevention (CDC)
- Germs and Hygiene — MedlinePlus, U.S. National Library of Medicine (NIH)
EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.
Researched 2026-08-22
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