Long before Edward Jenner lived, and even longer before viruses were discovered, people knew that you got certain infections only once. If you survived, you would probably never get that disease again.

When smallpox was the deadly killer, they used to try and infect healthy people with smallpox taken from someone having a mild attack. This didn't always work, because the newly infected person was just as likely to have a serious attack and die, or even worse, infect a lot of other people who would also die.

Edward Jenner's breakthrough was observing that people exposed to cowpox, a very mild disease, didn't get smallpox. Using this information, he developed and tested a successful vaccine, and published his results in 1798.

Since then, scientists have found that the best way to combat viral epidemics is with a vaccine. Vaccines teach the body to recognise the disease and kill off the microbes before they do any real damage.

Ever since a virus was linked to AIDS in 1983, scientists have been trying to find a vaccine to prevent HIV infection. But we don't have a vaccine yet, and many people are asking why.

We have every reason to believe that a vaccine is possible. Studies in sex workers, who are at high risk of HIV infection, show that some of them seem to be immune to the disease. Scientists analysing their blood found that those who didn't develop HIV, all had a very high level of a particular kind of white blood cell, a cytotoxic T-lymphocyte. If they could develop a vaccine that would stimulate the body to make more of those particular cells, we could well find the elusive cure for AIDS.

However, that's not quite as easy as it sounds, despite years of experience in developing successful vaccines for almost every viral infection known to man.

One of the problems is the large number of subtypes of HIV. Anyone who develops an immunity to one subtype may still get the disease from another subtype.

The other problem is that HIV is deadlier than smallpox. In order to make a vaccine, normally you have to use either some or all of the original virus, or else find a similar virus that produces a mild form of the disease. So far, I don't think we've seen any mild forms of AIDS. And while traditional vaccines use a weakened or dead form of the virus, I doubt that many people would agree to being injected with HIV, whether it was dead or alive or even kicking feebly.

Given these risks, scientists have been forced to find new ways of designing vaccines. With HIV, the traditional ways of making a vaccine are just too risky. The challenge is to find a way of getting the body to respond to a potential infection without any risk of infection from the vaccine. This, of course, opens up whole new areas in technology and scientific discovery.

What we need today, is another Edward Jenner who will find the "cowpox" of HIV.

In the absence of a similar but milder version of the virus, scientists are trying to make one themselves. Some of the vaccine models take a harmless version of a virus of a completely unrelated disease and engineer it genetically so that it produces HIV proteins which are not infectious. The immune system will learn about HIV from these proteins, so that if the real virus should enter the body at a later stage, it will know how to deal with it promptly and efficiently.

Other models are trying out small pieces of protein from various parts of the virus, to find out which ones produce the right immune response from the body. A vaccine using a small, harmless piece of protein taken from the outer covering of virus is currently in the final stages of research.

Of course, if one vaccine does turn out to be successful, research has to start again using proteins or genetic engineering to match a completely different subtype of HIV.

Or, scientists would have to find one part of the virus which is the same in all the variations of the virus. That's as hard as trying to find a cure for the common cold, which has over 100 different version of the virus.

There is no doubt that a vaccine preventing HIV infection and AIDS is possible. And it's quite clear that when it is discovered, it will have as much impact on the future of medicine as Edward Jenner's discovery of the vaccine that eventually wiped out smallpox.