How infections spread

To stop an infection, you need to know how it moves. Every infection that passes from one person to another follows the same basic path: an infectious agent leaves a source, travels by some route, enters a new person and finds that person able to be infected. Infection control practitioners describe this path as the chain of infection. Each link in the chain is a place where a control can break it.

This lesson explains the six links, the main routes of transmission and the timing of infection, from exposure to symptoms. It focuses on what matters for workplace controls. The biology of bacteria, viruses, fungi and parasites is covered in BIOL-108, lesson 7, and basic laboratory biosafety in BIOL-107, lesson 4.

The chain of infection

The infectious agent is the microorganism or virus that causes the disease: influenza virus, norovirus, Staphylococcus aureus, hepatitis B virus, Mycobacterium tuberculosis and so on. Agents differ in how many particles it takes to cause infection (the infectious dose), how long they survive outside the body, how resistant they are to disinfectants and how severe the illness is. Norovirus, for example, needs only a few particles to infect someone and survives for days on surfaces, which is why it spreads so explosively in care homes and on cruise ships.

The reservoir is where the agent lives and multiplies. For most workplace infections the reservoir is people: sick people, people in the incubation period who are not yet ill, and carriers who never feel ill at all. Other reservoirs include animals (for example, poultry and dairy cattle for avian influenza), water systems (Legionella in cooling towers, hot water systems and decorative fountains), soil and contaminated equipment.

The portal of exit is how the agent leaves the reservoir: the respiratory tract when someone breathes, talks, coughs or sneezes; the gut through vomit and stool; blood through wounds or needles; skin lesions; and other body fluids.

The mode of transmission is how it travels. This is the link most workplace controls target, and the next section describes it in detail.

The portal of entry is how it gets into the new host: inhaled into the nose, throat and lungs; through the mucous membranes of the eyes, nose and mouth; swallowed; through broken skin, cuts or needlesticks.

The susceptible host is a person who can be infected. Susceptibility depends on immunity from vaccination or past infection, age, pregnancy, chronic illness, medications that suppress the immune system and even fatigue and stress. A worker with eczema on their hands has more portals of entry than a worker with intact skin.

Modes of transmission

Many agents use more than one route. Influenza spreads mostly through respiratory particles at close range but can also spread by touching contaminated surfaces and, in crowded, poorly ventilated rooms, over longer distances. Norovirus spreads through the fecal-oral route, through contaminated surfaces and through vomit particles that become airborne when someone vomits. Controls need to match all the important routes, not just one.

Droplets, aerosols and an evolving understanding

For decades infection control divided respiratory transmission sharply into droplet spread (large particles that fall within about one to two meters, or three to six feet) and airborne spread (tiny particles that float). The COVID-19 pandemic forced a rethink. People exhale a continuous range of particle sizes, from large droplets to very small aerosols, whenever they breathe, talk, sing or cough. Larger particles fall quickly; smaller ones stay aloft and build up in a room unless ventilation removes them. Most transmission still happens at close range, where the concentration of exhaled particles is highest, but in crowded, poorly ventilated rooms, small particles can infect people farther away.

Public health agencies have updated their language to reflect this. Many now speak of respiratory transmission through short-range and longer-range aerosols, and they emphasize ventilation and well-fitting masks or respirators alongside distance. The older droplet and airborne categories remain useful for deciding which precautions to use for a particular disease in health care, as lesson 10 explains, but workplaces should not assume that a short distance is enough in a stuffy room. Lesson 6 covers ventilation.

Timing: from exposure to illness

Infections follow a timeline that matters for control. After exposure, there is an incubation period before symptoms appear. For influenza it is usually one to four days; for norovirus, 12 to 48 hours; for measles, about 10 to 14 days; for hepatitis B, weeks to months. People are often infectious before symptoms start, which is why screening for symptoms alone cannot stop every case. The infectious period is the time during which a person can pass the agent on; for measles it starts about four days before the rash and lasts until about four days after it.

How likely is infection after an exposure?

Whether an exposure leads to infection depends on the dose received, the route, the agent and the person's immunity. One brief contact may carry a small chance of infection, but repeated contacts add up. If each contact carries an independent probability p of infection, the chance of at least one infection after n contacts is one minus the chance of escaping every time.

# Cumulative chance of infection after repeated contacts (illustrative values of p).
for p in (0.01, 0.05):
    for n in (1, 5, 10, 20, 50):
        risk = 1 - (1 - p) ** n
        print(f"p = {p:.2f} per contact, {n:2d} contacts: {risk:5.1%}")

p = 0.01 per contact,  1 contacts:  1.0%
p = 0.01 per contact,  5 contacts:  4.9%
p = 0.01 per contact, 10 contacts:  9.6%
p = 0.01 per contact, 20 contacts: 18.2%
p = 0.01 per contact, 50 contacts: 39.5%
p = 0.05 per contact,  1 contacts:  5.0%
p = 0.05 per contact,  5 contacts: 22.6%
p = 0.05 per contact, 10 contacts: 40.1%
p = 0.05 per contact, 20 contacts: 64.2%
p = 0.05 per contact, 50 contacts: 92.3%

The probabilities in this model are illustrative, and real exposures are not independent or identical, but the lesson holds: small risks repeated many times become large risks. A receptionist who greets a hundred people a day during flu season, or a nurse caring for the same infectious patient over a week, faces far more risk than a single contact suggests. Controls that cut the risk per contact, such as ventilation, hand hygiene and masks, pay off because they apply to every contact.

Colonization, carriers and resistant organisms

Not everyone who carries an organism is sick. Many people carry Staphylococcus aureus in their noses without symptoms; this is called colonization. Colonized people can still spread the organism. Antibiotic-resistant organisms such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE) and carbapenemase-producing organisms (CPO) are a growing concern in health care because infections are harder to treat. They spread mainly by contact, so hand hygiene and cleaning are central to stopping them. Overuse of antibiotics drives resistance, which is why antibiotic stewardship programs are part of hospital infection control.

Breaking the chain in practice

Most workplace programs use several controls at once, each breaking a different link. Vaccination makes hosts less susceptible. Staying home when sick removes a reservoir from the workplace. Covering coughs and dressing wounds blocks portals of exit. Hand hygiene, cleaning and ventilation interrupt transmission. Gloves, eye protection and masks protect portals of entry. Because no single control is perfect, layering them, sometimes described as the Swiss cheese model, gives much better protection than any one alone.