Strains of the AIDS virus have developed a resistance to key medications used to keep infected people healthy, and doctors said Wednesday that the new strains may be impossible to treat.

AIDS viruses long have circulated that are impervious to AZT and other older AIDS medicines. But now people are beginning to catch viruses that also are resistant to protease inhibitors, which have revolutionized care of the disease in the past two years.A report released Wednesday at the 12th International AIDS Conference in Geneva said medications on the market may not be able to treat people infected with the resistant viruses.

Some people believe that the availability of AIDS treatment with protease inhibitors has made some people less diligent about safe-sex practices to prevent infection.

"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 National Institute of Allergy and Infectious Diseases.

No one knows yet how common these untreatable strains are. At the Geneva conference, San Francisco doctors reported one new infection with a highly resistant virus; a Swiss team said they had seen several more.