The flu is out in force. And so far this season, it’s been hitting children the hardest.

Influenza is a wily virus—it’s almost impossible to predict where and how it will strike as it circumnavigates the globe. There are, however, some patterns that routinely occur, allowing investigators and public health agencies to sometimes anticipate what lies ahead.

This is not one of those times.

Officials at the Centers for Disease Control and Prevention said flu-like symptoms started spurring higher than normal doctor visits in the U.S. in early November—weeks earlier than other recent flu seasons.

The disease has also shown up in and shut down elementary schools across the South and West, in states such as Texas, Idaho, Oregon and Alaska. Much of the Northeast, a traditional hotbed for flu-induced misery, remains largely unscathed for now.

At the same time, the severity of this year’s flu has been comparatively low. The number of people dying from pneumonia and influenza during the first week of December was substantially lower than the 6.4% threshold used to declare a flu epidemic at this time of year. (A new report on the spread of influenza spread is scheduled for release later Friday.)

This of course is a welcome change from two years ago, when an intense flu season started picking up speed in November and held a sustained peak into January and February. Some hospitals were so overwhelmed that they set up triage tents in parking lots. Ultimately, about 61,000 Americans died, making it one of the longest and most deadly flu seasons in years.

So why do so many people have the flu so early this year, and why is it relatively weak? The type of virus that’s circulating may be the explanation.

“Influenza activity is a little bit unusual for this time of the year because what we have predominantly is influenza B,” said Lynnette Brammer, head of the CDC’s domestic surveillance program. “Influenza B activity tends to impact children more than adults, particularly older adults. And the elderly drive mortality and hospitalizations.”

But influenza B doesn’t let the youngsters off the hook.

“If we stick with an influenza B season, I expect this would probably be a mild year for the population as a whole,” Brammer said. “But for kids, hospitalizations and even pediatric deaths would be similar to any other season. For kids, influenza B can be just as bad as influenza A.”

It’s uncommon for influenza B to hit first in the U.S., which often sees an initial wave of the more dangerous influenza A viruses—the types that can cause a flu pandemic. But the season is far from over in North America: The current spread of a weaker Influenza B could easily be followed by a strain of influenza A.

“Anyone who tries to predict the flu season based on early information doesn’t understand influenza very well,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “The case numbers are up early, but it’s such a difficult disease to predict—I don’t think we can say anything about how severe it may be.”

There is one point of which experts are certain, though.

“Flu is here,” Osterholm said. “Now is the time to get an immunization if you haven’t already.”

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