It's been a couple of years since the valley's seen a major outbreak of pertussis, and health officials are hoping to keep the number of cases down.
Pertussis, or whooping cough, that contagious respiratory illness marked by a "whooping" sound when a patient coughs, as well as fever and flu-like symptoms, was once nearly wiped out in the US.
But this year, there have been more than 18,000 cases reported to the centers for disease control.
In Washington state, health clinics are flooded with patients looking for a pertussis booster shot.
3,000 cases have been reported there.
That's triple last year's amount.
Mary Selecky, WA Dept. of Health: "It's really important for teenagers and all adults to get the booster shot."
It's when babies contract pertussis that doctors get worried.
Parents who notice whooping cough symptoms should have their child checked out right away.
Dr. Tom Clark, Epidemiologist: "It's of epidemic proportions. This is the largest number of cases we've seen since the 1950's."
Many cases are with 10 year olds who got the whooping cough vaccine as children, but haven't yet received the booster shot recommended at age 11.
But experts are also seeing a high number of cases in young teens who got the booster.
They say the shot still works, but maybe not as well as they thought.
But it seems to working in Fresno county.
There have only been three reported cases so far this year in Fresno county.
Compare that with 27 in 1999, and the peak year of 2010, when there were 532 cases.
72 were reported last year.
David Luchini, Fresno County Health Dept.: "I think the other thing that's helped also is doctors are more aware now of pushing the T-DAP vaccine to their patients. Hospitals have done a much better job at emergency rooms."
The Fresno County Health Department wants to remind everyone who has a child entering middle school, to have their T-DAP vaccines done. It's required by law.
The CDC also recommends all mothers and caretakers get the booster shot.