This is a question concerned parents are still asking in doctors' surgeries and baby clinics across Britain, seven years after Dr Andrew Wakefield's controversial study, purporting to link MMR with autism. Many parents ask 'isn't it better to expose our children to the viruses to induce immunity? After all many children contracted measles, mumps or rubella before vaccination programs existed and lived to tell the tale'. Outlined below are some of the reasons why we had all three of our children (2 boys and a girl) vaccinated, starting with a reminder of the symptoms and past generations' experiences of these diseases.

Prior to 1968, when the single measles vaccine was introduced in the UK, most people contracted this highly infectious virus, with peak levels of infection at primary school age. Measles is spread when droplets of the virus are exhaled by coughing or sneezing and inhaled by another individual. Seven to ten days later the child will begin to fell unwell with cold like symptoms - a fever, runny nose and cough, and then a rash appearing first on the forehead and gradually spreading down the face and across the body. This is sometimes accompanied by conjunctivitis. Complications can include middle ear infections, pneumonia or convulsions and one child in 5,000 will get encephalitis, an inflammation of the brain, which can be fatal.

Although not as infectious as its relative the measles virus, the mumps virus can also be spread through droplet infection and through direct contact with saliva. Two to four weeks later the salivary glands become swollen and painful with associated symptoms including fever, headache, earache, loss of appetite and abdominal pain. Although mumps is rarely fatal (killing around five people a year in the UK, before the MMR vaccine was introduced), meningitis (an inflammation of the lining of the brain and spinal cord) occurs in a small number of cases. In addition, boys who catch the disease in puberty can become sub-fertile and pregnant women with mumps are at risk of miscarriage.

The virus that causes rubella ('German measles') is not related to the measles virus. However, it is also spread by droplet infection, with symptoms appearing two to three weeks after infection. The disease itself is often quite mild and in young children there may be no obvious sign of infection. When symptoms appear they are: a runny nose and slight fever, swollen glands in the neck and a loss of appetite. A rash appears on the face and spreads across the body. By far the biggest risk with a rubella infection is the effect on unborn babies. For example, an older child, potentially with an undetected infection, can infect its unprotected mother carrying a sibling. Congenital rubella syndrome can result in a child born blind or with heart defects.

All the viruses used in the combined vaccine are living but attenuated (weakened) versions of the original, such that they cannot replicate and are therefore not infectious. However, they will stimulate the body to produce the required immunity, stimulating the production of antibodies. These are key defensive components of the immune system and, providing the levels are high enough, protect the individual from any future exposure to the virus. In this way, the vaccine stimulates the natural lifelong immunity gained by infection with the virus, without the symptoms or risks associated with actually having the diseases.

One of the reported concerns about the MMR is the supposed 'onslaught' on the immune system from the combination of three live viruses; but the immune system is designed to do just that, constantly coping with more than one challenge at any one time.

In addition, the components of the vaccine come into play at different times: 7-10 days, 3-4 weeks and 6 weeks after the injection for measles, mumps and rubella respectively. This can occasionally (one in 15,000 individuals) be associated with a localised swelling at the injection site and mild fevers and rashes.

The vaccine is given at 12-15 months and again at 3-5 years old, prior to school entry. The second immunisation is to protect those 5-10% of individuals who do not produce enough antibodies to be effectively protected by the first vaccine. Although everyone is encouraged to be vaccinated, there will always be a section of the population who cannot. An example is those individuals who are undergoing chemotherapy for cancer which compromises the immune system. This is one of the reasons that the guideline for the vaccination level necessary to maintain the immunity of the general population, the 'herd immunity', is set at 95%. Similarly, with the increasing trend towards exotic holidays in countries where measles, for example, is endemic, the risks of infection for an unprotected child may be much higher than in the UK.

The MMR is one of the most widely used vaccines in the history of vaccination clocking up over 30 years of use in the US, 20 years in Sweden and Finland, 15 years in the UK and over 10 years in the rest of Europe. This has been accompanied by a dramatic decrease in the number of babies with congenital rubella syndrome and a substantial reduction in the sickness and deaths caused by mumps and measles.

However, alarm bells have rung, principally in the UK, about the potential unwarranted effects of the vaccine itself. These fears were triggered by a press conference in 1998 that focused on the findings of gut specialist Dr Andrew Wakefield and his colleagues. They suggested that there was a link between the measles component of the MMR vaccine and the development of a particular form of autism, associated with a bowel disorder. It is important to point out that this particular study only involved 12 patients and the authors themselves stated that 'we did not prove an association between MMR vaccine and the syndrome described'. Since then, no research group has been able to verify Wakefield's findings. Numerous large scale analyses have also shown no link between the incidence of autism and the MMR vaccine, a good example being the Finnish experience involving 3 million children over a period of 14 years.

The causes of autism are not at all clear. The effects of disorders that are classified under the umbrella of autism can be devastating for the child and family concerned. Most likely autism, like many conditions, arises as a result of the interaction between genetic and environmental factors. The incidence is apparently increasing, perhaps due to more efficient diagnosis, with one in 166 children in the UK now considered to have some form of autism. Plans are afoot to screen all pre-school children for autistic type disorders in the light of concerns that not enough is being done to discover the causes of autism.

Although Dr Wakefield raised concerns about the MMR vaccine, he and others support the principle of vaccination. Single vaccines have attracted a lot of press as an alternative route to protecting children. However, only the rubella vaccine (routinely used until 1996) is licensed for use in this country. Legally, the use of single vaccines is restricted because the view is that there is a safe and effective alternative in the form of the MMR vaccine. Although they may well be effective vaccines in their own right, there are several issues related to their use. The number of individuals who have received the single vaccines is much lower than those receiving the MMR, thus making it less likely that any potential side-effects will be detected. Similarly, the recommended, but arbitrary spacing of one year between single vaccines leaves a child susceptible to contracting these diseases in the interim and makes parents less likely to follow through with the 6 injections necessary, versus 2 for the MMR.

There are strong arguments now for presenting concerned parents with the facts about the MMR vaccine, rather than anecdotal reports stirred up by media coverage. Science does not allow for definitive statements that there are no risks involved, as any form of medical intervention is associated with risks, however minimal. But, the evidence presented by respected scientists and health professionals in both the UK, and other countries that use the MMR vaccine, supports its continued use. Although, it has been suggested by some that the vaccine should be compulsory, as it is the US, the ethos of the NHS emphasises the need for informed consent. If parents understand the facts about the MMR vaccine, perhaps previous generations' experiences of measles, mumps and rubella will remain a thing of the past.