Although most children are vaccinated several times against pertussis, those shots wear off with age. It is possible, the authorities said, that a new, safer vaccine introduced in the 1990s gives protection that does not last as long, so more teenagers and adults are vulnerable.

And, Dr. Poland said, if many New Yorkers are catching laryngitis, as has been reported, it is probably a rhinovirus. “It’s typically a sore, really scratchy throat, and you sometimes lose your voice,” he said.

Though flu cases in New York City are rising rapidly, the city health department has no plans to declare an emergency, largely because of concern that doing so would drive mildly sick people to emergency rooms, said Dr. Jay K. Varma, deputy director for disease control. The city would prefer people went to private doctors or, if still healthy, to pharmacies for flu shots. Nursing homes have had worrisome outbreaks, he said, and nine elderly patients have died. Homes need to be more alert, vaccinate patients, separate those who fall ill and treat them faster with antivirals, he said.

Dr. Susan I. Gerber of the C.D.C.’s respiratory diseases branch, said her agency has not seen any unusual spike of rhinovirus, parainfluenza, adenovirus, coronavirus or the dozens of other causes of the “common cold,” but the country is having its typical winter surge of some, like respiratory syncytial virus “that can mimic flulike symptoms, especially in young children.”

The C.D.C. and the local health authorities continue to advocate getting flu shots. Although it takes up to two weeks to build immunity, “we don’t know if the season has peaked yet,” said Dr. Joseph Bresee, chief of prevention in the agency’s flu division.

Flu shots and nasal mists contain vaccines against three strains, the H3N2, the H1N1 and a B. Thus far this season, Dr. Bresee said, H1N1 cases have been rare, and the H3N2 component has been a good match against almost all the confirmed H3N2 samples the agency has tested.

About a fifth of all flus this year thus far are from B strains. That part of the vaccine is a good match only 70 percent of the time, because two B’s are circulating.