CNN HEALTH Special Section

GENEVA (CNN) -- Doctors reported for the first time Wednesday the ominous spread of a strain of the AIDS virus that is resistant to protease inhibitors, the medicines that have revolutionized care of the disease.

At the 12th International AIDS Conference, San Francisco doctors reported one new infection with a highly resistant virus. A Swiss team said it has seen several more. No one knows yet how frequently these strains are spreading.

AIDS viruses impervious to AZT and other, older AIDS medicines have long circulated. But now people are beginning to catch viruses that are also resistant to protease inhibitors, which are the pivotal ingredients of the drug cocktails that have made AIDS a survivable disease.

The infections will almost certainly be difficult and perhaps impossible to treat, at least with the medicines on the market now.

"This is a wake-up call to people who assume that since we have adequate therapy, if they get infected they will be easily treated," said Dr. Anthony Fauci, head of the U.S. National Institute of Allergy and Infectious Diseases.

"It's like going back to the early 1980s when we had no therapy, because when a virus is resistant to everything, it's just like it was when we had no therapy."

The San Francisco case was described by Dr. Frederick Hecht of the University of California, San Francisco. A report on it will be published later this month in the New England Journal of Medicine.

"What happened here is somebody who was on therapy and who was not properly treated, (was) inconsistently treated, developed a resistant virus and transmitted this virus to another person," said Tom Coates of the UCSF AIDS Research Institute.

The new patient, a middle-aged San Francisco man, caught the virus last fall. He was infected by a man whose own infection was diagnosed in 1990 and had taken many different AIDS drugs on and off, including protease inhibitors.

The San Francisco man's virus is resistant to four different protease inhibitors as well as the drugs AZT and 3TC.

Hecht said doctors first became concerned about the patient after he responded much more slowly than usual to drug treatment. The man stopped therapy entirely after learning his virus was resistant, but he is still healthy.

The Swiss cases were reported at the conference by Dr. Sabine Yerly and others from University Hospital in Geneva.

Since the beginning of treatment with three-drug cocktails, doctors have been concerned about the spread of resistant strains. Following the regimen means taking 15 or 20 pills a day on a precise schedule, and missing even a few doses allows mutant viruses resistant to protease inhibitors to emerge.

Another AIDS expert cautions that the appearance of the resistant strain is not a cause for panic.

"It's happening but it's rare, and it would be wrong to say that this is a rampant phenomenon. It's not, but it's the beginning," said Dr. David Ho of the Aaron Diamond AIDS Research Center.