The virus, called HTLV-II, is normally harmless, but has become rife among drug abusers who are spreading it between each other much more rapidly than would normally be the case in the general population.

Scientists believe the increased transmission rate has prompted the development of the virus into a potentially more harmful form.

Several drug users infected with the virus have already developed a serious neurological disorder.

The virus is believed to have spread from monkeys to humans between 15,000 and 30,000 years ago in the Americas. The virus is also common in the pygmy and American Indian tribes, and is normally transmitted through breast-feeding.

Contaminated blood

But in recent years the virus has reached a larger population through contaminated blood in shared needles. The virus has been detected among drug users in the US, Asia and Europe, with an infection rate of between five and ten per cent.

New Scientist magazine reports that the researchers compared the rate of growth of the virus between drug users in Italy, Ireland, the US, Spain, Sweden and Vietnam with American Indian tribes.

They discovered that the genetic sequence of HTLV-II developed at a rate of one per cent every 100,000 years in the tribes, whereas in drug users the rate was 300 times faster.

The scientists, Marco Salemi and Anne-Mieke Vandamme at the Rega Institute in Leuven, Belgium and Professor William hall of University College, Dublin, believe that HTVL-II stays dormant most of the time and only grows in the short time when it is attempting to invade a new host's cells.

Although breast-feeding does not spread the virus very efficiently, a drug user can spread the virus to hundreds of other users within a few months. During this time, the virus is able to multiply even more.

Professor Hall is to carry out a further study of drug users serving long-term prison sentences in Brazil to see if the virus is becoming more likely to cause disease (pathogenic).

"The virus was first recognised in drug users in the late 1980s, and up to 15 per cent of users in Ireland are thought to be infected with the virus," he told News Online.

"If its is evolving at this rate, it could become more pathogenic in future years. We really just at the stage of identifying the prevalence of the disease and why it mutates so rapidly."

Professor Hall said the link had been established between the virus and serious neurological disease, but that this occurred only in a small minority of cases.