It is not your imagination - more people you know are sick this winter, even people who have had flu shots.

The country is in the grip of three emerging flu or flulike epidemics: an early start to the annual flu season with an unusually aggressive virus; a surge in a new type of norovirus; and the worst whooping cough outbreak in 60 years - all against the backdrop of the normal winter highs for the many viruses that cause symptoms on the "colds and flu" spectrum.

Influenza is widespread and causing local crises. On Wednesday, Boston's mayor declared a public health emergency as cases flooded hospital emergency rooms.

Google's national flu trend maps, which track flu-related searches, are almost solid red (for "intense activity"), and the Centers for Disease Control and Prevention's weekly FluView maps, which track confirmed cases, are nearly solid brown (for "widespread activity").

"Yesterday, I saw a construction worker, a big strong guy in his Carhartts who looked like he could fall off a roof without noticing it," said Dr. Beth Zeeman, an emergency room doctor for MetroWest Medical Center in Framingham, just outside Boston. "He was in a fetal position with fever and chills, like a wet rag."

Nationally, deaths and hospitalizations are still below epidemic thresholds. But experts do not expect that to remain true. Pneumonia usually shows up in national statistics only a week or two after emergency rooms report surges in cases, and deaths start rising a week or two after that, said Dr. Gregory A. Poland, a vaccine specialist at the Mayo Clinic in Minnesota.

The predominant flu strain circulating is an H3N2, which typically kills more people than the H1N1 strains that usually predominate; the relatively lethal 2003-4 "Fujian flu" season was overwhelmingly H3N2.

No cases have been resistant to Tamiflu, which can ease symptoms if taken within 48 hours, and this year's flu shot is well-matched to the H3N2 strain, the CDC said. Flu shots are imperfect, especially in the elderly, whose immune systems may not be strong enough to produce enough antibodies.

The CDC and 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.