Federal health officials yesterday asked states to consider tracking people who have had contact with U.S. residents who may be infected with a mysterious new lung ailment, which would be a significant escalation of efforts to contain the spread of the disease domestically.

The announcement came after the federal Centers for Disease Control and Prevention in Atlanta received a report that a Florida man may have spread the potentially fatal disease to a co-worker, which would mark the first time in the United States that the illness has spread outside close contacts.

Federal health officials cautioned that neither the Florida man's infection nor his co-worker's illness has been confirmed as severe acute respiratory syndrome (SARS). All prior evidence indicated that SARS is spread mainly among people who have traveled in parts of Asia where the infection is most common, their family members or health care workers who have cared for them.

If the Florida transmission is confirmed, it would raise the disturbing possibility that the disease can spread more easily. Federal officials said they began stepping up their efforts to ensure that everything possible was being done to contain the disease.

"It is possible for this virus to very quickly spread under certain circumstances," said CDC Director Julie L. Gerberding, adding that officials don't know "whether that's because there are modes of transmission such as the airborne route that we haven't identified or documented well yet, or because some people are particularly efficient transmitters."

"We have to be prepared for the possibility that we will have spread outside the close family or health care workers. That's why we are taking steps today to enhance our monitoring of contacts," she said.

The CDC issued guidelines for monitoring students at schools with suspected SARS cases. It also organized a teleconference with state and local health officials to discuss more aggressive efforts to track the contacts of other suspected cases.

Currently, anyone who has had contact with a suspected SARS patient is urged to watch for symptoms and notify authorities if they fear they are infected. The CDC is considering new recommendations that state and local health officials identify anyone who has had contact with a possible case and watch to see if they become ill.

"We will know not only passively if a contact develops any illness but we will be proactively monitoring them . . . where their symptoms can be recognized at the earliest possible moment so they have very little chance of transmitting it to anyone else," Gerberding said.

At least 166 cases of suspected SARS in 30 states are being investigated in the United States, including three in Virginia. Virginia officials said yesterday a fourth suspected case had been identified. Most cases have been people who recently returning from Asia, but three health care workers who cared for patients and nine family members have been infected by the travelers.

Health officials have been meeting planes, boats and cargo ships from Asia, isolating sick travelers and asking fellow passengers to look out for symptoms. Anyone suspected of having the disease is being isolated to reduce the risk of it spreading.

No one has died from SARS in the United States, and so far the disease has appeared to be milder in this country than in Asia.

At least 2,781 people in 18 countries are suspected of having contracted SARS, and 111 victims have died. Most of the cases have been in China, and most infections have occurred among family members and hospital workers.

But there have been several mysterious clusters that have alarmed public health officials. Hundreds of residents of a 33-story apartment Hong Kong building, for example, were evacuated to quarantine camps when the virus started spreading from apartment to apartment in unknown ways. Another mysterious cluster is being investigated at a Singapore hospital.

The Florida case, one of 11 suspected cases in that state, involved someone who recently returned from Asia and developed symptoms consistent with SARS. Subsequently, a co-worker developed similar symptoms.

"There's certainly no indication of spread beyond that point," Gerberding said. "The [state] health department is aggressively taking the appropriate steps to make sure they have communicated with all of the exposed people or potentially exposed people. . . ."

The CDC has also begun consulting with the Food and Drug Administration about blood donations from people who might be at risk for SARS, to prevent them from donating blood. Although there's no evidence SARS can be spread through blood transfusions, it might be prudent to discourage donations from high-risk people, Gerberding said.

The CDC announcement came as the New England Journal of Medicine rushed onto its Web site two scientific papers describing the evidence that a previously unknown type of coronavirus is the cause of SARS. Viruses of this type usually cause nothing more serious than the common cold. CDC scientists recommended naming the virus after Carlo Urbani, the World Health Organization scientist who alerted the world about the emerging epidemic before he succumbed to it.

The CDC also announced that preliminary testing by military scientists indicated that the antiviral drug ribavirin, which some doctors had thought might be effective in treating SARS, did not look promising.

Special correspondent Catharine Skipp contributed to this report.

CDC director Julie Gerberding says a Florida case indicates that SARS can sometimes spread rapidly.