Host-pathogen interaction and transmission
Introduced| English | Meaning |
|---|---|
| chain of infection | Agent, reservoir, exit, transmission, entry and susceptible host: breaking any link stops spread. |
| carrier/ˈkærɪə/ | A well person who carries and sheds a pathogen and so can pass it on without being ill. |
| colonisation | The presence of a microbe living on or in the body without causing disease. |
| serial interval | The time from one case's symptoms to the next case's: measles about 11 days, influenza 2–3 days. |
| basic reproduction number | R₀: the average number of new cases one case causes in a fully susceptible population. |
54 guests out of 120
- Two days after a wedding, 54 of the 120 guests have vomiting and diarrhoea. Who caught it, from what, and how do you stop the next case?
- Infection follows rules you can draw and compute: a chain with five links, and a number that sets how fast it spreads.
The chain of infection
- The chain of infection 感染链 has five links: a reservoir, a way out, a route, a way in, and a susceptible host.
- Break any one link and transmission stops; every control measure targets a link.
Put the links of the chain of infection in order.
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The microbe leaves its reservoir, travels, enters and meets a host who can be infected.
Five routes
- Contact (hands, surfaces), droplet (large drops within 1–2 m), airborne (tiny particles that hang in the air), vector (mosquitoes, ticks) and vehicle (food, water).
- The route decides which precaution works.
Match each infection to its main route.
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The route decides the precaution.
Colonised, carrier or ill
- A microbe can live on a person without harm (colonisation 定植), be carried and shed by a well person (a carrier 带菌者), or cause disease.
- Reservoirs need not be ill: a healthy typhoid carrier can infect many people through food.
A reservoir of infection must be a person who is ill.
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Healthy carriers, animals, food and surfaces can all be reservoirs.
R₀ and the serial interval
- The basic reproduction number 基本再生数 $R_0$ is the average number of people one case infects in a fully susceptible population: measles 12–18, influenza about 1.3.
- The serial interval 代间隔 is the time from one case's symptoms to the next case's: measles about 11 days, influenza 2–3 days. Early on, cases multiply by $R_0$ every serial interval.
What does R₀ measure?
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R₀ measures spread, not severity.
Doubling time
- Doubling time: $t_d \approx \mathrm{SI} \times \dfrac{\log 2}{\log R_0}$. Influenza ($R_0$ 1.3, SI 2–3 days) doubles in about 5–8 days; measles ($R_0$ 15, SI 11 days) in about 3.
- Isolation, vaccination and distancing lower the effective R, the real number in a real population, and slow the doubling.
An infection has R₀ = 4 and a serial interval of 4 days. Using $t_d \approx \mathrm{SI} \times \log 2 / \log R_0$, what is the doubling time in days?
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log 2 / log 4 = 0.5, so 4 × 0.5 = 2 days.
Worked example: the wedding buffet
- Attack rate: 54/120 = 45%. Split by one dish: 85% of seafood eaters fell ill, against 10% of those who skipped it.
- Relative risk: 85% ÷ 10% = 8.5. The seafood is the vehicle; one fraction, split once, is the first page of an outbreak investigation.
85% of people who ate a dish fell ill, against 10% of those who did not. What is the relative risk?
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85% ÷ 10% = 8.5.
Worked example: breaking a ward outbreak
- (Fictional) Diarrhoea spreads on a ward. Reservoir: patients and contaminated surfaces. Way out: stool. Route: hands and surfaces. Way in: the mouth. Hosts: other patients.
- Hand washing breaks the route, cleaning removes the reservoir, and grouping cases together protects the susceptible hosts. The diagram is the plan.
Which link of the chain does hand washing break?
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Hands carry the microbe from one person to the next; washing removes it on the way.
Three common slips
- "A high R₀ means a deadly disease." Spread and severity are separate.
- Using R₀ when the effective R applies. Immunity and behaviour change the real number.
- Forgetting carriers. Well people can be the reservoir.
Recap
- Five links: reservoir, way out, route, way in, susceptible host. Break one to stop spread.
- $R_0$ with the serial interval sets the doubling time, $t_d \approx \mathrm{SI} \times \log 2 / \log R_0$.
- Attack rates split by exposure name the vehicle.