Horizon tells the microbiological detective story in which some of the best brains in science have been pitted against the most extraordinary bug the world has ever seen.

In 1984 it was discovered that HIV was the cause of AIDS. Straight away, there were confident predictions that there would be a vaccine ready for testing in just two years.

Back then, just 1,292 deaths from AIDS had been reported. Now the figure is 25 million dead. By 2010 it is predicted there will be 85 million infections and 70 million deaths. And after 20 years there is still no sign of a vaccine.

Despite work of dazzling complexity, the ambition of so many brilliant scientists has been constantly thwarted. Just as a vaccine seems to be working, the AIDS virus alters itself, and ten to fifteen years of work, and millions of pounds, go down the drain. These bitter disappointments are only compounded by the desperate human urgency of the work.

This is a story where the clock doesn't stop ticking.