Some medical “breakthroughs” are simply hot air. But one that really has transformed our lives is the near-conquest of infectious disease.

Parents a century ago were regularly desolated by the death of children, often from diseases that today are quickly cured by antibiotics, or prevented altogether by vaccines.

But success has made us complacent. Infections are making a come-back, helped by the misuse of antibiotics and the reluctance of some parents to have their children fully vaccinated. This week’s news of a measles outbreak is a small but potent reminder of how easily things can go wrong.

Why has measles reappeared?

It’s always out there. According to the World Health Organisation, measles caused 454,000 deaths in 2004. So the virus responsible is hardly lying low. It is ready to infect children and spread, given the right conditions.

Relatively speaking, there have been only a handful of cases in Britain and a single death, of a 13 year-old boy. It spreads easily and rapidly among those who have not had it before, or have not been vaccinated.

In extreme cases, it can be terrifying. An epidemic in Fiji in 1875, which affected a population unfamiliar with the virus and hence extremely vulnerable to it, is believed to have killed about a quarter of the population.

Who is at risk?

Those at greatest risk are children over the age of 6 months and old enough to be mixing regularly with other children who have not been vaccinated twice with MMR (measles, mumps and rubella) vaccine. Measles is one of the easiest diseases to catch – spread by coughs and sneezes – and an infected child may pass it on to others before symptoms even become apparent. In a house with a single measles patient, 90 per cent of people who lack immunity will catch the disease.

Isn’t it just a harmless childhood infection?

This is a view taken by the complacent. No disease that causes fever is entirely trivial, even though the vast majority of healthy children will recover unharmed. But the mortality caused by measles in the Third World shows that it is not to be taken lightly.

What are the symptoms?

Fever for at least three days, combined with at least one of the following: cough, runny nose and red eyes. A rash appears first on the head and later spreads to the rest of the body, causing itching.

What should parents do now?

Nothing, if they have been sensible enough to ensure that their children have been given two doses of MMR. A single dose is better than nothing, but provides incomplete protection.

If children have not been vaccinated, the Health Protection Agency recommends that they should be, as soon as possible. “It is crucial that children are fully immunised with two doses of MMR before they return to school” said Mary Ramsay, consultant epidemiologist with the agency.

Why isn’t vaccination compulsory?

In some countries it is. In the US, for example, children cannot enter the state school system unless they have been fully vaccinated.

In Britain a more flexible system, dependent on the sense and goodwill of individuals, has developed. GPs and health visitors will try hard to ensure that children are vaccinated, but it is not compulsory. This is to allow parents with religious, ethical, or downright daft antipathies to vaccines to exercise their freedom of choice. Admirable as this policy may be, it has its drawbacks.

For example?

In the late 1970s, research that later turned out to be false linked the whooping cough vaccine to brain damage in children. Millions of parents calculated that they would rather take the risk of whooping cough than brain damage, and opted out of vaccination. The result was a fall in vaccine coverage from about 80 per cent to fewer than 40 per cent, and a corresponding increase in whooping cough cases from almost zero in the mid1970s to peaks of more than 60,000 a year in 1979 and 1982. It is estimated that between 60 and 100 children died who need not have done. Since then, vaccine use has risen to near 100 per cent and cases of whooping cough are down to a few hundred a year.

Given this history, why have we made the same mistake again?

Good question. In 1998 a paper in The Lancet reported on 12 cases of an unusual bowel condition in children, apparently linked to autism. The paper mentioned the belief by some of the parents of the children that symptoms had appeared soon after the children had been vaccinated with MMR.

One of the authors of the paper, Andrew Wakefield, had long believed in a link between MMR and autism. The paper, and his subsequent campaign, cause a media scare that repeated, in close detail, what had happened two decades earlier with whooping cough.

Parents began to fight shy of MMR, and vaccination rates fell. Even the publication of a series of convincing rebuttals could not halt the panic.

Why was the scare believed, and not the rebuttals?

Most parents actually rejected the scare and – perhaps with some trepidation – allowed their children to be vaccinated. Rates of MMR uptake never fell as far as they did with whooping cough uptake in the 1970s. But there were parents who felt that measles was unlikely to kill or injure their child, but that MMR might. They were sincere in this belief, and found backing from Dr Wakefield, parts of the media and from private clinics that sprang up around the country to provide single vaccines.

Weren’t they entitled to go for single vaccines, if they wanted?

They were, but there was actually less evidence of the safety of single vaccines than there was for MMR, which protects against measles, mumps and rubella. The big clinical trials had been done with MMR, not with single vaccines.

And giving a child single vaccines means that he or she has to visit the clinic six times, not twice. Children do not enjoy vaccinations, and their parents do not like to see them distressed. So the result was that the whole course of vaccination often went uncompleted, leaving children vulnerable to one or more of the diseases.

But aren’t we jabbing our children to death?

Far from it. The aim of the childhood vaccination programme is to protect children against as many diseases as possible, with the minimum number of visits to the clinic.

Children’s immune systems are exposed to challenges from millions of foreign materials and are amply capable of responding to them all. There is no evidence of “vaccination overload”.

So is measles about to run rife in Britain?

Probably not. About three quarters of children have had two doses of MMR, which limits the disease’s target population.

While it is not high enough to ensure “herd immunity” this level of vaccination should limit the scale of any epidemic.

Are any other diseases making a comeback?

This week a group of experts started a campaign to reawaken awareness of the value of vaccines. They gave warning that typhoid, a waterborne infection that used to cause huge epidemics, has increased by 69 per cent in the past five years and a milder version, paratyphoid, by 78 per cent.

While numbers are still low, even a single case of typhoid that had contaminated the recent floods in middle England might have led to a serious outbreak. Typhoid still kills 600,000 people around the world each year.

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