Infectious disease: transmission and control

Transmission and the points at which it can be interrupted

An infectious disease persists only if its pathogen passes between hosts. Control therefore targets one or more parts of the transmission chain: the source of infection, the route between hosts, or host susceptibility. The most effective combination depends on the pathogen and its route of transmission.

The examples below are cholera (faecal-oral transmission), malaria (a mosquito vector), tuberculosis (airborne transmission) and HIV (infected body fluids). For each disease, distinguish prevention of transmission from treatment of an infected person.

The routes fall into a short list. Direct contact and body fluids: HIV passes only when infected fluid meets a mucous membrane or broken skin. Droplets: coughs and sneezes launch aerosols that another person inhales, which is how tuberculosis and influenza move through a shared room. Water and food: the faecal-oral route, where sewage reaches something that will be drunk or eaten, carries cholera. Vectors: an organism, usually biting, ferries the pathogen from blood to blood. Some pathogens add durable spores that wait in soil or dust for years, which is how tetanus enters a cut and how fungal diseases cross a field.

Transmission

The passage of a pathogen from an infected host to an uninfected one, directly or through an intermediate such as water, food or a vector.

Vector

An organism that carries a pathogen between hosts without suffering the disease itself; the female Anopheles mosquito is the vector of malaria.

Carrier

An infected person with no symptoms who still sheds the pathogen and can pass it on.

Endemic

Describes a disease permanently established in a region or population, as malaria is across much of the tropics.

Living conditions multiply every route at once. Overcrowding shortens the distance a droplet has to travel; missing sanitation connects sewage to drinking water; malnutrition weakens the immune response that would otherwise end an infection early. These diseases therefore concentrate in the poorest places, which is why control measures are classified on an economic axis as well as a biological and a social one.

Cholera: faecal-oral transmission

Vibrio cholerae leaves one host in faeces and enters the next by mouth. Where sewage and drinking water are kept apart, cholera is close to impossible; where a flood, an earthquake or a war camp mixes them, one infected arrival can seed an epidemic within days, because a single water source serves thousands of mouths.

Two features increase transmission. Many infected people are carriers, shedding the bacterium with mild symptoms or none, so the number of sources exceeds the number of visible cases. And the watery diarrhoea produced by the toxin, several litres a day, contains large numbers of V. cholerae, which reaches the water supply where sanitation is absent.

The classic demonstration that control does not need a cure is John Snow's, in London in 1854, thirty years before the bacterium was identified. He mapped the cholera deaths of Soho, found them clustered around one public water pump in Broad Street, and had the pump handle removed. The outbreak collapsed. Snow knew nothing about the organism; he had found the route.

Modern control is the same insight with better tools. The engineering that separates sewage from drinking water is expensive once and effective for a century. Oral vaccines exist and work, though protection fades within a few years, so they are used to blunt outbreaks and protect travellers rather than as a permanent shield. And for whoever is infected anyway, oral rehydration solution turns a disease that can be fatal within a day into one that is usually survived, at very low cost. Prerequisite: the cholera toxin, osmosis and oral rehydration.

Malaria: transmission by a mosquito vector

Malaria is caused by four species of the protoctist Plasmodium: P. falciparum, P. vivax, P. ovale and P. malariae. Falciparum is the dangerous one, responsible for most deaths; vivax and ovale can hide dormant in the liver and relapse months or years later; malariae runs the mildest, longest course. Name the four species and identify P. falciparum as the one causing most severe disease.

The parasite cannot pass from person to person. It needs two hosts, and the second is the female Anopheles mosquito, female because only females take blood meals. Biting an infected person, she takes up the parasite's sexual stages with the blood. They complete their development in her gut, and when she next feeds, one to two weeks later, the infective stages travel in with her saliva. In the person they multiply first in the liver, then in red blood cells, whose synchronised bursting gives the periodic waves of fever.

A two-host cycle gives control two targets, and the vector is the easier one. Insecticide-treated bed nets exploit the fact that Anopheles feeds mostly at night: the net blocks the bite, and the insecticide kills the mosquito that lands on it. Indoor spraying does the same for the walls she rests on after feeding. Draining or covering standing water removes the still pools her larvae need, and stocking ponds with larvae-eating fish is biological control in the strict sense, one organism deployed against another.

Resistance has developed to each antimalarial drug in turn. Artemisinin, isolated from sweet wormwood, is given as a combination with a second drug on purpose: a parasite carrying a mutation against one drug is still met by the other, so resistant lines are far slower to establish. Vaccines reached the clinic only in the 2020s and protect partially, because Plasmodium moves through liver, blood and mosquito stages that each present different antigens. The antigens therefore change with life stage as well as by mutation, which is a more difficult case than influenza's antigenic variation.

Tuberculosis: two species, two routes

Tuberculosis is caused by Mycobacterium tuberculosis, and also by Mycobacterium bovis, whose usual hosts are cattle. The two organisms give the disease two routes, and each route has its own control.

M. tuberculosis travels in droplets. An untreated person with active TB coughs the bacterium into shared air, and infection is a matter of dose and time: hours in the same poorly ventilated room, night after night, is the classic setting, which is why the disease tracks overcrowded housing, prisons and shelters. Most people infected never become ill; the bacterium survives inside macrophages, walled into granulomas, and waits. Malnutrition, age or an untreated HIV infection can release it decades later, which is one of the ways the two pandemics feed each other.

M. bovis travels in unpasteurised milk and close contact with cattle. The control is agricultural rather than medical: herds are tested and infected animals slaughtered, and pasteurisation heats milk enough to denature the bacterium's proteins. In countries that do both, M. bovis disease in people has almost disappeared, an entire transmission route closed without a single patient treated.

Treatment exists and is long: a combination of antibiotics taken for six months or more, because the bacterium grows slowly and shelters inside cells. The length of the course creates a risk of resistance. A patient who stops at two months, when symptoms have gone, leaves the least susceptible bacteria in the infection, which arose by chance mutation, and the infection that relapses is resistant to those drugs. Multi-drug-resistant TB is the accumulated result, and it is why control programmes fund supervised courses, with a health worker watching each dose swallowed, rather than handing over a bottle of tablets. The BCG vaccine gives partial protection, strongest against severe childhood forms, so it contributes to a control programme without preventing all infection.

HIV: transmission in body fluids

The human immunodeficiency virus passes only in body fluids: unprotected sex, transfused blood and shared needles, and from mother to child in pregnancy, birth or breast milk. It does not survive drying, is not carried by insects and does not cross intact skin, so shaking hands, sharing cups and sharing air do not transmit it. Much of the stigma attached to the disease has followed from inaccurate beliefs about these boundaries.

Each route has a corresponding control. Condoms place a barrier between fluids. Donated blood is screened, subject to the testing-window limitation noted with ELISA. Needle exchanges supply sterile equipment to people who inject drugs, and substantially reduce transmission. Antiretroviral drugs given in pregnancy cut mother-to-child transmission from roughly one in four to under one in a hundred.

Antiretroviral drugs do not clear the infection, because the virus integrates its genome into the T-helper cells that would coordinate a response against it; replication resumes if the drugs are stopped. They hold replication low enough that the immune system remains intact and AIDS does not develop, and a person with a sustained undetectable viral load does not transmit the virus sexually, so treatment also functions as prevention.

There is at present no vaccine. The virus mutates its surface antigens rapidly within a single infected person, and it infects the T-helper cells on which a vaccine response would depend. Prerequisite: antigenic variation and the specific immune response. With no cure and no vaccine, control acts on transmission, and global estimates of new infections have fallen since a peak in the 1990s.

Biological, social and economic control measures

Every measure described above is biological, social or economic, and control programmes that rely on one category alone are less effective.

Biological measures act on the pathogen or its vector: vaccines, drugs, insecticides, fish that eat larvae, pasteurisation. Social measures change what people do and where they live: sanitation, ventilation, condom use, contact tracing, testing and slaughtering infected herds, education. Economic measures decide whether the first two happen at all: a net costs a few pounds and a sewage system costs millions, and the populations that need them most can afford them least.

The axes interlock rather than add. A bed net is a biological object; whether a family sleeps under it every night is social; whether they own one is economic. A six-month antibiotic course is biological; finishing it is social; being able to miss work for clinic visits is economic. Disease and poverty act on each other in both directions: illness reduces work and schooling, so expenditure on control has an economic return as well as a health benefit.