Why do the viruses in mumps vaccines not cause mumps?

The viruses in mumps vaccines are live attenuated strains, meaning they have been weakened so they cannot cause the disease in healthy individuals. These modified viruses can still stimulate an immune response, allowing the body to recognize and fight off the actual mumps virus if encountered in the future. The attenuation process ensures that while the immune system is trained to respond, the vaccine does not lead to the symptoms of mumps.