It's back to school Monday for many spring breakers, but even before vacation, schools like Silver Lake Elementary in Everett saw 10 percent of its student body out sick.

Snohomish County Health District officials can't say for sure what's behind the high number of absentees. They are tracking a health epidemic countywide: perstussis, also known as whooping cough.

It's a tremendously serious disease for us and we're definitely in a middle of an epidemic, said Tim McDonald, Snohomish County Health District spokesman.

Whooping cough has hit Marysville the hardest in Snohomish County; 69 cases have been reported so far this year - 32 in Arlington and 25 cases in Everett.

That's definitely the tip of the iceberg - many more cases out there beyond the confirmed, said McDonald.

Health officials said the way to bring down these alarming numbers is to get vaccinated.

The immunity to pertussis wanes considerably 5 to 6, 7 to 8 years your immunity goes down to 0, said McDonald.

During this health epidemic, experts stress all teens and adults need to get the T-DAP vaccine to prevent spreading it to the mroe suspectible infants and young children.

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