Measles, mumps, rubella and varicella (also known as chickenpox) are infectious diseases caused by viruses. They are most common in children and young adults, and can lead to potentially fatal illnesses, disabilities and death. Measles remains one of the leading causes of childhood death around the globe. Rubella is also dangerous for pregnant women, as it can cause miscarriage or harm to unborn babies. The MMR (measles, mumps, rubella) is a combined vaccine that protects against all three infections. Clinicians can vaccinate against varicella (chickenpox) by a combined MMR and varicella vaccine (MMRV) or giving it separately at the same time (MMR+V).