FOLSOM, Pennsylvania (WPVI) -- The Pennsylvania Health Department says as of today, there are 13 cases among students and staff at the Delaware County school.
The first 5 cases were announced last Thursday.
Vaccination rates at the school are very high, but immunity can wane over time in some people.
And some recent studies indicate the prevailing virus has drifted from what's in the vaccine.
Numerous outbreaks have been reported in recent years in schools and colleges.
Last week, a notice went out to Ridley students reminding them "Don't share eating utensils or drinking straws" and "Wash hands thoroughly and frequently."
Delaware County Senior Medical Advisor Dr. George Avetian says mumps is spread through saliva or respiratory secretions.
It causes a few symptoms to many viruses, like fever, malaise, headache.
And the telltale sign - swollen salivary glands.
"So you will see the glands in the front of ear by jawline puff out," said Dr. Avetian.
Over 4,800 students and staff vaccinated against mumps during Temple clinics.
13 mumps cases now tied to Ridley High School, health Department says