Dallas County Health and Human Services officials sent out a new alert Thursday afternoon.
They're now reporting 33 cases of mumps so far this year and 27 of those are at Cedar Hill High School.
County health is sending teams out to the school on Friday to offer a third booster shot for free.
The latest news from around North Texas.
Experts say there's still a roughly 12 percent chance of getting the mumps even with two vaccines, and with a high number of cases in a concentrated area, it can spread quickly.
Parents at Cedar Hill High School are feeling a little over-protective right about now.
"If she coughs or sneezes or something, I do ask her if she's feeling alright," said parent Deborah Hickey.
They’re worried over news that 27 students and staff at the school have come down with the mumps.
"I'm concerned that the numbers are growing, even though the school says they're cleaning,” said Hickey. “I think they may need to bring in some professional cleaners."
Everyone who's gotten sick at the school reports that they have been vaccinated, but like any vaccine, it's not 100 percent effective.
DCHHS is now recommending folks at the school get a third vaccine to boost their immunity during the outbreak.
But experts say that does not mean anyone else outside the school needs a third shot.
"It's not an all or nothing situation where you're protected by three, but you weren't protected by two,” said Dr. Michael Sebert, Medical Director of Infectious Disease at Children’s Health Dallas and an Associate Professor at UT Southwestern. “Most people are protected by two and that's why we've been very successful as a public health community here in the United States keeping mumps at very low levels with vaccination."
As parents now weigh getting their kids that third booster, many are pushing the school to bring in an outside company to clean as thoroughly as possible.
"The first person who got it, that should have been an alarm that we need to exterminate our school immediately," said grandmother Shirley Daniels.
A district spokesperson told NBC 5 they are not planning to bring in an outside company because they already have professional cleaners on the job. They're using a stronger disinfectant and scrubbing down to the light switches and door knobs.
But the mumps is most likely to transmit person-to-person in the moment of that cough or sneeze.
The district says there are a few families at Cedar Hill High who signed waivers to not have their child vaccinated. Those students have now either gotten a vaccine, or left the school during the outbreak.
North Texas has been experiencing an increase in mumps cases over the past few months, said DCHHS, including unrelated outbreaks in neighboring counties.
An ongoing outbreak in Johnson County is beginning to subside, according to Dr. Elvin Adams, of Johnson County.
If you'd like to learn more about the mumps, visit the Dallas County Health and Human Services website.