How Infections Spread

TL;DR. Every infectious disease is a solution to one engineering problem: get from inside one host to inside the next. The route it uses (air, water, sex, blood, mosquito, touch) determines nearly everything else about it: who catches it, how fast it spreads, which season it peaks in, which countries it devastates, and which intervention stops it. Sanitation ended cholera, not antibiotics. Bed nets cut malaria more than any drug. Masks and ventilation matter for tuberculosis and not at all for hepatitis C. If you know the route, you can usually predict the control measure without knowing any microbiology at all.

Key takeaways

• Five kinds of pathogen (viruses, bacteria, fungi, parasites, prions) differ so much that "germ" is close to useless as a category. Antibiotics work on one of the five.
• The transmission route predicts the control measure. Water route means engineering; vector route means the vector; sexual route means behaviour, barriers, and treatment-as-prevention.
• R0 is the average number of people one case infects in a fully susceptible population. It sets the herd immunity threshold, the fraction that must be immune to stop sustained spread: $1 - 1/R_0$. Measles at R0 near 15 needs about 93 percent; that is why measles is the first disease to come back when vaccination slips.
• The gap between infectiousness and symptoms decides whether isolation can work. SARS was controllable because people were sickest when most infectious. COVID-19 was not, because they were not.
• Roughly 60 percent of human infectious diseases came from animals. New ones keep arriving by the same door.
• Killing the host fast is usually bad strategy for a pathogen, which is why extremely lethal outbreaks often burn out locally while mild ones circle the globe.

The five kinds of pathogen

In short: Antibiotics work on one of the five, which is the entire reason they do nothing for a cold.

Two consequences of that table drive the rest of the book. Antibiotics do nothing to viruses, so taking them for a cold gives you all the risk (side effects, gut damage, resistance selection) and none of the benefit. See Chapter 36. And viruses are hard to drug because they use your machinery, so there is little to attack that is uniquely theirs. That is why medicine leans so heavily on vaccines for viruses and drugs for bacteria.

The routes, and what stops each

In short: The transmission route predicts the control measure, which is why sewers ended cholera and bed nets cut malaria more than any drug did.

A pathogen must exit one host, survive transit, and enter the next in sufficient number (the infectious dose, which ranges from about 10 organisms for Shigella to hundreds of thousands for some others). The route it uses is the most informative single fact about it.

This table is why history reads the way it does. London's cholera did not end because someone cured cholera. It ended because John Snow traced the 1854 Broad Street outbreak to one water pump, and because London then spent decades building sewers. Cutting the route beats treating the disease, almost every time, and it beats it at lower cost.

Don't be confused: "airborne" and "droplet" used to be a hard distinction, and that distinction was wrong. The old teaching held that large droplets fell within about two metres while only a few special diseases (measles, TB, chickenpox) travelled as true aerosols. Aerosol physics established during COVID-19 that exhaled particles form a continuum, that small ones stay suspended and accumulate in poorly ventilated rooms, and that the two-metre rule is a rough guide rather than a boundary. This is why ventilation became a serious public health topic for the first time in a century.

Timing: the numbers that decide whether control is possible

In short: If people become infectious before they feel ill, isolating the visibly sick cannot work, which is the whole difference between SARS and COVID-19.

Four intervals define an infection's behaviour, and their relative order matters more than their absolute lengths.

• Incubation period: infection to first symptom.
• Latent period: infection to becoming infectious.
• Infectious period: the window during which you can transmit.
• Serial interval: the average gap between one case's symptom onset and the next case's.

Now the crucial comparison. If you become infectious after you feel ill, symptoms are a usable alarm: people isolate before they spread. If you become infectious before symptoms, or without ever having them, isolation based on symptoms leaks.

Rabies is the extraordinary case: the incubation is so long that vaccinating after the bite still beats the virus to the brain. Almost no other infection allows this.

R0, and why measles is the canary

In short: Measles needs roughly 95 percent immunity to stop spreading, so it is always the first disease to return when vaccination coverage slips.

R0 (pronounced "R-nought") is the average number of secondary cases produced by one infectious case in a completely susceptible population. It is not a fixed property of a microbe; it depends on biology, behaviour, and setting. The same pathogen has a higher R0 in a crowded city than in a village.

If R0 is above 1, cases grow. Below 1, the outbreak dies out. Rt, the effective reproduction number, is the same quantity in the real population at time t, with immunity and interventions included. Every control measure is an attempt to push Rt below 1.

The herd immunity threshold is the immune fraction needed to hold Rt below 1:

$$ H = 1 - \frac{1}{R_0} $$

Two lessons fall out. Measles is the most contagious common human disease, so it needs the highest coverage, so it is always the first outbreak to return when vaccination rates dip a few points. It is an early-warning indicator for a whole immunisation programme. And herd immunity is a threshold, not a gradient of personal safety: below it, outbreaks are possible; the protection it offers to people who cannot be vaccinated (infants, the immunosuppressed) only exists once the threshold is cleared.

R0 also explains why smallpox could be eradicated (moderate R0, no animal reservoir, obvious rash, stable virus, effective vaccine) while flu cannot be (animal reservoirs in birds and pigs, constant mutation, presymptomatic spread).

Why the deadliest outbreaks are often the smallest

In short: Pathogens evolve toward maximum transmission rather than toward mildness, so lethality is selected against only when it interferes with spreading.

There is a widespread intuition that pathogens evolve toward mildness. The truth is more specific: pathogens evolve toward maximum transmission, and lethality is selected against only when it interferes with transmission.

Ebola kills a large share of the people it infects and requires close contact with body fluids. Patients become too ill to move around while at their most infectious, so outbreaks tend to burn out locally unless they reach a city. A rhinovirus that gives you a runny nose keeps you at work sneezing on people, and it circulates worldwide forever. The 1918 influenza pandemic is the awkward counterexample: trench and troop-transport conditions moved even severely ill patients efficiently, which removed the usual penalty on virulence.

Note the corollary: a pathogen with a long asymptomatic infectious period can be both lethal and extremely successful. Untreated HIV kills nearly everyone it infects and still spread to tens of millions, because it spent years being transmissible and invisible.

Where new diseases come from

Roughly 60 percent of known human infectious diseases and about three-quarters of recent emerging ones originated in animals. The pattern of spillover is consistent: sustained close contact between people and animals, an amplifying setting, then a virus that manages the jump.

HIV crossed from primates in Central Africa in the early twentieth century. Influenza pandemics assemble in pigs and birds. Ebola persists in bats. SARS reached humans through live-animal markets in 2002, MERS through camels, and SARS-CoV-2 most probably through an animal intermediary at a market in Wuhan, a question still formally open.

The drivers of spillover are not mysterious: expanding agriculture into wildlife habitat, dense livestock farming, wildlife trade, and air travel that converts a local outbreak into a global one within a week. This is why surveillance networks watch animal populations, and why influenza vaccine strain selection is an annual international process.

Sources and notes

Transmission categories, incubation periods, and R0 ranges are drawn from standard references (Heymann, Control of Communicable Diseases Manual; CDC and WHO fact sheets) and from the epidemiological literature; R0 values vary substantially between studies and settings and are given as ranges deliberately. The measles R0 range of 12 to 18 comes from classic pre-vaccine data and has been argued upward and downward since. The John Snow Broad Street investigation is 1854. The zoonotic-origin proportions come from Taylor, Latham, and Woolhouse (2001) and Jones et al. (2008). Aerosol transmission physics: the reassessment during 2020 to 2021 by Wang, Marr, Morawska, and colleagues.

Open questions. The origin of SARS-CoV-2 remains under investigation and is not settled. Herd immunity thresholds calculated from a single R0 assume uniform mixing, which no real population has, so the true figures are approximations.

Next: the other source of disease, the one you are born with. 👉