The measles, mumps, and rubella (MMR) vaccine is a combination vaccine that protects against three contagious diseases: measles, mumps, and rubella. It is typically administered to children in two doses, with the first dose given around 12 to 15 months of age and the second dose between 4 to 6 years. The MMR vaccine works by stimulating the immune system to produce antibodies against these viruses, helping to prevent infection. Vaccination is crucial for public health, as it reduces the spread of these diseases and protects those who cannot be vaccinated due to medical reasons.