NORTH PHILADELPHIA (WPVI) -- The Philadelphia Health Department says there are now 23 cases tied to the mumps outbreak reported at Temple University.It's the city's largest outbreak of the mumps since 2011, but it may not due to students skipping vaccines.Unfortunately, we have seen outbreaks of the mumps popping up over the past several years. In fact, there were 15 outbreaks last year including ones at Penn State University and Harvard.One reason is because the vaccine that protects against the virus can start to wear off over time.The recent mumps outbreaks are more commonly seen on college campuses due to students being in close contact.Some people infected will have no symptoms, while others can experience fever, headache, loss of appetite and fatigue, plus the tell-tale sign: swollen salivary glands.Complications are rare but can be serious. They include inflammation of the brain, hearing loss and male infertility.And the mumps virus is very contagious."It's a very contagious disease and if you put a lot of college kids together in a dorm, it can easily be transmitted to bunches of different students," says Dr. John Russell, program director of family medicine at Abington Hospital, Jefferson Health.It's spread through saliva and respiratory secretions (mucus). People remain contagious for about 9 days after symptoms start.Temple University doesn't require proof of mumps vaccination but plans to start doing it with the 2019 incoming freshmen.Most children receive two doses of the vaccine before the first grade. Russell says one can still contract the disease despite being vaccinated."What do you say to critics who say that's why I'm not getting my child vaccinated?" asks Action News reporter Christie Ileto."We see a certain amount of herd immunity. If the vaccine isn't effective in everyone, but everyone has been vaccinated, than everyone is protected," said Russell.The CDC also recommends a third dose of the vaccine for high risk groups such as people in close contact to an infected person.On college campuses, that includes roommates, fraternity and sorority members, and athletic team members.