It's a reminder, they said, of how genetically flexible and cunning the human immunodeficiency virus can be.

The new strain so far has been found only in one patient, a 40-year-old woman in Cameroon who died of AIDS in 1995. In the past two years, new HIV strains also have been discovered in Thailand and India. However, they were more conventional variants that did not have close links to non-human primate viruses.

French and African officials have launched a public health investigation in Cameroon and neighbouring Gabon to determine if the new strain is being widely transmitted.

Infectious disease experts are publishing a report on the new strain in the September one issue of Nature Medicine. They said they do not expect it to become prevalent, but it could escape detection by current diagnosticmethods used in laboratory screening programmes.

``The present isolate is rather a rare bird,'' said Simon Wain-Hobson of the Institute Pasteur in Paris, who did not participate in the study.

HIV constantly evolves into new strains, even as researchers develop new combinations of therapies to control its proliferation.

On a global scale, there are two simultaneous epidemics, HIV-1 and HIV-2. HIV-1 is the more widespread of the two viral groups HIV-2 is mainly in Asia and East Africa.

Most of the strains in HIV-1 belong to a group designated as M, for major strains. There is also an O group for minor, outlying strains that appear to be clustered in West Africa.

The new strain in Cameroon is HIV-1 but is neither an M or an O. Instead, the study's lead author, Francois Simon of the Hospital Bichat in Paris, said the new strain veers noticeably from both established groups and deserves the new label of N.