MODEL RELEASED. MMR vaccine. Nurse filling a syringe with the MMR vaccine. This is a combined vaccine to protect infants from 3 viral diseases: measles, mumps and rubella. The first injection is given in the infant's second year. The vaccine consists of weakened samples of the three viruses, causing a weakened response from the body's immune system. This response prepares the immune system so that it can defend against any future infection by these 3 viruses. All three diseases can have serious effects, but measles is potentially fatal. Vaccines are given to babies and young children as part of a childhood vaccination schedule. Combined vaccines speed up this process.