05
The Chain of Transmission
Agent, Host & Environment, and the six links that connect them
Every infectious disease outbreak involves three interacting forces, known as the epidemiologic triad:
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Agent
The pathogen or substance that must be present for disease to occur. It can be biological (a microbe), chemical (lead, carbon monoxide) or physical (heat, radiation, the force of a crash).
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Host
The person or animal that can become infected: their immunity, age, and behavior all matter.
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Environment
The outside conditions (sanitation, climate, crowding, food handling) that bring agent and host together.
Why the triad is useful: Changing any one side of the triangle can stop disease. Treating the agent (disinfecting), protecting the host (vaccination), or fixing the environment (better sanitation) can all break the outbreak.
Zoom in further, and infection actually travels through a specific sequence of six links called the chain of infection. Break any single link, and transmission stops cold, which is exactly the point of most public health interventions.
👆 Tap any link to "break" it with a real control strategy, and see whether the infection can still get through.
✅ The chain is intact: infection can spread
Tap a link above to see an example control strategy for it.
Three Ways to Travel (Mode of Transmission)
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Contact
Direct (touching), indirect (via a contaminated object, or "fomite"), or droplet (a cough or sneeze at close range).
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Vehicle
A contaminated substance (food, water, blood) or object that can carry the agent to many hosts at once.
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Vector
A living carrier. Mechanical (a fly carrying germs on its body) vs. biologic (a mosquito where the pathogen actually develops inside it).
Droplet vs. airborne, by size: the two are split by particle size. Droplet particles are larger than 5–10 µm: they're heavy enough to fall to the ground within a few feet, so droplet spread needs close contact. Airborne particles are smaller than 5 µm: light enough to stay suspended and drift on air currents far beyond the person who released them.
CDC's version: direct vs. indirect. Direct transmission is direct contact (touching, kissing, sex, a bite) plus droplet spread at close range. Indirect transmission is airborne (tiny particles that hang in the air), vehicle-borne (food, water, blood, or objects called fomites) and vector-borne (mosquitoes, ticks, fleas). If a test asks "direct or indirect," airborne counts as indirect.
What Makes an Agent Dangerous?
Infectivity
How easily the agent can enter a host and multiply (become established).
Pathogenicity
The proportion of infected hosts who go on to develop clinical disease.
Virulence
How severe the disease is among those who become clinically ill: including the proportion of severe or fatal cases.
Optimal virulence: you might expect pathogens to evolve toward being as deadly as possible, but that usually backfires on them. A pathogen that kills or immobilizes its host too fast loses its ride before it can spread to anyone new. So natural selection tends to push virulence toward a middle ground: whatever level lets the pathogen spread the most, not whatever level does the most damage. That's part of why a pathogen spread by mosquitoes (which doesn't need a mobile host) can afford to be more severe than one that depends on a host walking around and shaking hands.
How flu changes from year to year: Antigenic drift is small, gradual mutations that build up over time. It's why last year's flu vaccine doesn't match this year's strain perfectly. Antigenic shift is a sudden, major change, often when two different flu strains (say, a human and a swine strain) infect the same host and swap genetic material, producing an entirely new subtype nobody's immune system has seen before. Shift, not drift, is what can trigger a pandemic: the 2009 H1N1 "swine flu" pandemic started this way.
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Q7In the chain of infection, what is the difference between a "portal of exit" and a "portal of entry"?
The portal of exit is how the agent leaves its reservoir (e.g., a cough); the portal of entry is how it gets into a new, susceptible host (e.g., the respiratory tract).