First the snow. Now the flu. Turns out, it's winter in Minnesota after all.

So far this season, the flu has largely been missing in action across the country. But now, in what officials say is the latest start to the flu season in 29 years, the number of confirmed cases is starting to climb.

Last week, some 40 Minnesotans were hospitalized with confirmed cases of flu, nearly twice the number the week before, according to the Minnesota Department of Health. And the number of confirmed school outbreaks rose from 32 to 38.

"We're kind of seeing it pick up for the first time," said Karen Martin, one of the official flu trackers at the department. "It's definitely a late season, for sure."

In a typical year, the flu would have been emptying classrooms and filling hospital beds since December, and hitting its peak in February.

Instead, "we're just now seeing what seems to be the onset of flu season," said Patsy Stinchfield, director of infectious disease at Children's Hospitals and Clinics of Minnesota.

As of mid-February, there were relatively few reported outbreaks in Minnesota schools (32) or nursing homes (7) since the start of the season; and fewer than 100 people hospitalized.

The trend was noticeable at Children's, which is typically flooded with flu cases by this time, Stinchfield said. This year, she said, only three children have been hospitalized with the flu.

No one is sure why the flu has been so scarce, though many speculate that the mild weather is one cause. That's probably only partly true, says Kris Ehresmann, director of infectious disease at the Health Department. The flu virus appears to thrive in cold, dry conditions, she said. And in harsh winters, "people tend to stay indoors more," where they're more likely to infect one another.

But this year, many people may simply be immune because the current flu strains have been circulating for two years, Ehresmann said. Unlike other years, she said, "we haven't seen the emergence of a new strain."

Yet a "milder year" doesn't mean that the flu virus has lost its power, she added. It can still can still make people miserable and trigger deadly complications; at least five people have died in Minnesota so far this year. The only thing that's different is that fewer people are getting infected.

Even so, experts warn, it's impossible to predict what the flu virus will do next. "I think it will probably continue to go up for a while," Martin said. "So get your shot if you haven't."

Stinchfield agrees. "We have not declared the flu season over," she said. "Influenza is a very wily virus."

Maura Lerner • 612-673-7384