SEATTLE (AP) — Health officials are reporting a huge upsurge in the number of flu cases in Washington state this month – a time of year when the flu season is usually winding down.

Record numbers of people have tested positive for influenza in Snohomish County this week, and five people have died of the flu in Kitsap County alone since the end of February. As of March 9, there were 93 lab-confirmed deaths from the flu statewide.

Health officer Jeff Duchin says this year’s late flu season is due to an unusual progression in the circulation of flu viruses.

During most flu seasons, influenza type A circulates first and is followed by an influenza type B later in the season. But flu is unpredictable, and this season began with influenza type A/H1N1, and now is being followed by a large, late surge in influenza A/H3N2.

And that’s a problem, he says, because the A/H3N2 strain causes higher rates of severe illness, hospitalizations and deaths than the other types.