NEW YORK — On the day after an announcement that a rare strain of the AIDS virus was found in a New York man, scientists said much work needed to be done to assess how dangerous the virus is.

But, they quickly added, anything that scares people away from using methamphetamine and having unprotected sex with strangers is a useful public-health measure.

Even doctors who joined New York health officials in making the announcement Friday said that it may have been misinterpreted, because it is impossible to say that an especially virulent bug is spreading when only one person is known to have it, and that person has been followed for only a few weeks.

"Every medical center in a major metropolitan area will have cases like this," said Dr. Roger Pomerantz, an AIDS specialist at Thomas Jefferson University in Philadelphia. "You've got to really prove something like this before you go on CNN and scream about a 'super strain.' "

Project Inform, a 20-year-old San Francisco group that provides information about acquired immune deficiency syndrome and treatment, called the reports "unnecessarily alarming to the public."

Investigators believe the infected man may have contracted the virus in October when he engaged in unprotected sex with multiple partners while using crystal methamphetamine.

Dennis DeLeon, president of the Latino Commission on AIDS, said it was common for men using methamphetamine to have sex with 10 to 20 partners in one night. "It is a drug where they just lose count," he said.

3 drugs ineffective

The man tested negative for human immunodeficiency virus, or HIV, in 2003, then tested positive in December. By last month, he was exhibiting signs of AIDS and it was clear that his infection was not responding to three of the four classes of anti-retroviral drugs used to treat HIV. In particular, experts said, it needs to be seen whether the same virus behaves as virulently in any other men infected by the New York man. It might be less dangerous in someone with a different genetic makeup or less of a drug habit. City health officials said only that they had two people trying to locate the infected man's sexual partners.

Dr. Anthony Fauci, a prominent AIDS researcher and head of the National Institute of Allergy and Infectious Diseases, said he had seen cases in which two sexual partners had been infected by the same strain and "one does reasonably well, and the other progresses fulminantly" — meaning the immune system of the second person fails rapidly.

What is not clear, several experts said, is whether the disease progressed so rapidly because the virus was strong or the patient was weak.

About 1 percent of all people infected are "slow progressors," who take decades to become sick. At the other end of the spectrum will be 1 percent to 2 percent who are "fast progressors," who go to full-blown AIDS — meaning low numbers of CD-4 immune-system cells and opportunistic infections such as Pneumocystis carinii pneumonia — in months. The average time between infection and AIDS is seven to 10 years.

The New York man, who is clearly a fast-progressor, has a "dual-tropic" strain of the virus, which means it can attach to two types of receptors, nicknamed the R5 and X4 receptors, on the outside of the CD-4 cell.

In a vast majority of infected people, said Dr. Robert Gallo, a co-discoverer of the AIDS virus, the first infections are attached to the R5 receptors. Only after several years do the X4 receptors kick in, and then the patient often goes downhill quickly.

In a small number of patients, the primary infection is on the X4 receptors, and they often are fast-progressors.

But such patients are seldom-studied, so it is not known why the virus attaches there. "It's probably by chance, or someone whose receptors are more active," Gallo said. "But it can be the patient, not the virus, that's rare."

Role of amphetamines

Pomerantz, who studies interactions between drugs and the virus, said there was some evidence that the infection moves more rapidly in alcohol and heroin abusers, but little is known about whether amphetamines, which are chemically very different, suppress the immune system or affect X4 receptors.

By contrast, Dr. Jeffrey Klausner, director of prevention of sexually transmitted diseases for the San Francisco Department of Public Health, said he thought amphetamine use could lower CD-4 cell counts and could have contributed to the speed of the New York man's infection.

Mutations that make a strain drug-resistant may make it less aggressive than the original strain, several experts said. The health officials who raised the alarm Friday said they did so because they thought the man was the first they had seen who was resistant to so many drugs and also a fast progressor.

But other experts repeatedly stressed yesterday that it was too early to determine how rare that condition was or even why it might exist in the New York man.

Maybe 2 strains at once

It is even possible, surmised Dr. Fred Valentine, an AIDS expert at New York University, that the patient was infected with two strains — one that ravaged his immune system, and another one that was drug-resistant. Drugs would have suppressed the devastating strain quickly, leaving only the drug-resistant one to take the blame for the damage.

Dr. Jack Dehovitz, a professor of infectious disease at State University of New York Downstate, who was among the public-health experts at Friday's announcement, said he was uncomfortable with the expression "super-strain."

"We've been dealing with resistant strains from the beginning of the treatment era," he said, acknowledging that the rapid progress in one patient "might be a coincidence."

The point of the public warning, Dhovitz said was to "encourage the community to do the right thing."

New York Times reporters Carol Pogash, Richard Perez-Peña and Marc Santora contributed to this report.