The rubella virus, a togavirus of the genus Rubivirus, is an enveloped single-stranded RNA virus with a single serotype that does not cross-react with other togaviruses. Humans are the only known host, with seasonal epidemics occurring every 5-9 years over a worldwide distribution. Rubella virus is transmitted by the respiratory route and the virus replicates in the nose, throat and the local lymph nodes. The incubation period ranges from 12 to 23 days, with an average of 18 days. Viraemia occurs 5-7 days after exposure and leads to viral spread to different organs.

In pregnant women the virus infects the placenta and the developing fetus. Infection by rubella virus is characterized by a transient rash. Joint pain and arthritis rarely occur in children, but may affect up to 70% of adults. Rubella virus can be found in nose and throat secretions and urine from 1 week before to 2 weeks after rash onset, although infants with congenital rubella may excrete the virus for a year or more in pharyngeal secretions and urine.

Congenital rubella syndrome is caused by infection in early pregnancy; from just before conception and during the first 8-10 weeks of gestation rubella infection may result in multiple fetal defects in up to 90% of cases, affecting many organs and often results in miscarriage or stillbirth. Infants with congenital rubella syndrome that survive may face serous developmental disabilities.

The first sign of measles is usually a high fever, which begins about 10 to 12 days after exposure to the virus, and lasts 4 to 7 days. A runny nose, a cough, red and watery eyes, and small white spots inside the cheeks can develop in the initial stage. After several days, a rash erupts, usually on the face and upper neck. Over about 3 days, the rash spreads, eventually reaching the hands and feet. The rash lasts for 5 to 6 days, and then fades. On average, the rash occurs 14 days after exposure to the virus (within a range of 7 to 18 days).

Most measles-related deaths are caused by complications associated with the disease. Serious complications are more common in children under the age of 5, or adults over the age of 30. The most serious complications include blindness, encephalitis (an infection that causes brain swelling), severe diarrhoea and related dehydration, ear infections, or severe respiratory infections such as pneumonia. Severe measles is more likely among poorly nourished young children, especially those with insufficient vitamin A, or whose immune systems have been weakened by HIV/AIDS or other diseases.