The 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, the first between 12 and 15 months of age and the second between 4 and 6 years. The vaccine helps prevent outbreaks and serious complications associated with these diseases. The measles virus can cause severe respiratory issues and complications like pneumonia, while mumps can lead to swelling of the salivary glands and potential infertility in males. Rubella, also known as German measles, can cause serious birth defects if a pregnant woman contracts the virus. The MMR vaccine is a safe and effective way to protect against these illnesses.