Doctors in the United States and Spain have reported an increasing number of patients who have all the symptoms of the illness - and many of the infections which are associated with it - but show no trace of the known forms of the virus in their blood.
There are two major types of the virus, HIV-1 which causes the majority of infections world-wide, and HIV-2.
Some of these HIV-negative Aids, cases as they are being called, were in high risk groups for infection; others had no known risk factors.
Thomas Spira, an epidemiologist at the US Center for Disease Control, in Atlanta, has identified six patients - two of whom had blood transfusions after screening for HIV had been introduced. Of the remainder, one was an intravenous drug user and one was a health care worker. The other had a long history of blood transfusions.
According to the report in Newsweek magazine, Dr Spira now believes that 'HIV may not be the only infectious cause of immunosuppression in man'.
Other health workers are concerned that another form of the virus would hinder their attempts to control the spread of Aids, according to the magazine.
A key factor in prevention of spread has been the ability to screen for HIV. Developing a blood test for a new viral agent would be vital.
In another case, a married man from Spain with no known risk factors for HIV developed Kaposi's sarcoma (a form of skin cancer common in Aids and tuberculosis). His white blood cell count had dropped to between 40 and 120 whereas the normal range is between 600 and 1,200. But despite two years of repeated testing no trace of HIV was found.
The cases of another three patients are now being followed up by the Spanish doctors.
Commenting on the report yesterday, Dr Gerald Myers from the Los Alamos National Laboratories in New Mexico, who is attending the eighth International Conference on Aids this week in Amsterdam, said that he 'had a hunch' that scientists would find that these patients were infected with a retrovirus - a virus belonging to the same family of HIV.
He said that HIV was a 'moving target, constantly threatening to slip out from under the net of detection techniques'.
Five sub-types of HIV-1 had already been identified, he said. There was evidence to show that the virus was changing in step with the spread of infection - that is, viral variation was epidemic led.Reuse content