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. When I see one of those cases, I just tighten up my mask a little.”

Massachusetts General Hospital in Boston started asking visitors with even mild cold symptoms to wear masks and to avoid maternity wards. The hospital has treated 532 confirmed influenza patients this season and admitted 167, even more than it did by this date during the 2009-10 swine flu pandemic.

Not an epidemic … yet

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 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.

Simultaneously, the country is seeing a large and early outbreak of norovirus, the “cruise ship flu” or “stomach flu,” said Dr . Aron Hall of the CDC’s viral gastroenterology branch. It includes a new strain, which first appeared in Australia and is known as the Sydney 2012 variant.

This week, Maine’s health department said that state was seeing a large spike in cases. Cities across Canada reported norovirus outbreaks so serious that hospitals were shutting down whole wards for disinfection because patients were being infected after moving into the rooms of those who had just recovered.

The classic symptoms of norovirus are “explosive” diarrhea and “projectile” vomiting, which can send infectious particles flying yards away.

“I also saw a woman I’m sure had norovirus,” Zeeman said. “She said she’d gone to the bathroom 14 times at home and four times since she came into the ER. You can get dehydrated really quickly that way.”

This month, the CDC said the United States was having its biggest outbreak of whooping cough, or pertussis, in 60 years; there were about 42,000 confirmed cases, the highest total since 1955.

The disease is unrelated to flu but causes a hacking, constant cough and breathlessness.

While it is unpleasant, adults almost always survive; the greatest danger is to infants, especially premature ones with undeveloped lungs. Of the 18 recorded deaths in 2012, all but three were of infants under age 1.

That outbreak is worst in cold-weather states, including Colorado, Washington, Wisconsin, Minnesota and Vermont.