flu

Influenza is especially intense this year, and people are flooding into hospitals and doctors' offices. But the flu is just one of a triple whammy of respiratory viruses — plus the nasty norovirus — that are making lots of people sick.

It's still too soon to say whether this is a historically bad flu season. But it's already clear that emergency rooms around the country are filled with feverish throngs that are much larger than during the last flu season.

As students return to class from winter break, campus health official are trying to avert an outbreak. Colleges in Boston are especially worried after the mayor's declaration last week of a public health emergency in the city.

Vaccination is still the best way to avoid getting sick, but it's not 100 percent protective. Some people may get infected with a strain of flu that isn't covered by this year's vaccine. For others, the shots just don't work.