With thousands possibly exposed to Mumps at a national cheerleading event in Dallas, Texas 2 weeks ago, making sure children are up to date on their vaccinations has never been more critical. People usually wonder why these events occur, discrediting herd immunity and vaccine efficacy. However, nothing is 100% in medicine. Even if everyone gets vaccinated, not everyone will respond and develop immunity. One or two may remain susceptible. Also, not everyone can get vaccinated as some can have true allergy to some vaccine components while some are immunecompromised to a point that vaccination is contraindicated. Therefore, these small number of people are dependent on the rest of the population being immunized to be protected from the disease. However, if there are other people not immune, like free riders, the disease can more easily spread. This gif beautifully illustrates this.