To have lived with HIV for 17 years makes Chris Smith one of the longer survivors of the virus that leads to Aids. Many people who became infected in the 1980s are now dead.

The first drug developed to tackle HIV in the scramble for treatments after the epidemic began in the USA and Europe was AZT.

It was toxic, with unpleasant side-effects, and suppressed the virus for only a short period: HIV is a dangerous enemy, mutating quickly and frequently to overcome new drugs. In many people the virus rapidly developed a resistance to AZT.

But other drugs were developed by the pharmaceutical companies. Although none can wipe out HIV from the body, doctors found in the 1990s that patients would survive for a long time with combinations of drugs.

The standard treatment in the west is now highly active antiretroviral therapy, or Haart, which is a combination of three different drugs.

The pills have to be taken every day or else the virus will fight back, but Haart is keeping people well, able to hold down demanding jobs and lead normal lives.

"There are a number of people who have lived for 20 years or more with HIV," said Lisa Power, the head of policy at the Terrence Higgins Trust.

Chris Smith's example was "a very, very good demonstration that if you get your HIV diagnosis and get the best treatments, you can live for a long time.

"There aren't many jobs that are more stressful than cabinet minister".

It is important to eat well and to stay fit. Mr Smith has done both over the years.

Over nearly two decades, he will undoubtedly have had to change the combination of drugs he has taken. Even when the drugs are well adhered to, resistance creeps in and new ones are needed.

Science is just about keeping up with the virus, and new types of medicines are constantly being developed to keep it at bay, although a cure looks as far away as ever and we are nowhere near a vaccine to prevent more people becoming infected.

Mr Smith is fortunate to be living in an affluent part of the globe. Basic antiretroviral combinations are only now becoming available to a minority in sub-Saharan Africa, where the pandemic is at its most severe, thanks to the campaigning of activists.

Out of 6 million people in the region in urgent need of the drugs, only 700,000 have recently begun taking them.

This has become possible thanks to a dramatic fall in the price of the drugs, but the second- and third-line combinations that will be needed later are still prohibitively expensive.