Measles morbillivirus, formerly called measles virus (MeV), is the cause of measles. Humans are the natural hosts of the virus; no animal reservoirs are known to exist. Both rubella, also known as “German measles”, and roseola are different diseases caused by unrelated viruses.
Initial symptoms typically include fever, often greater than 40 °C (104 °F), cough, runny nose, and inflamed eyes.
Small white spots known as Koplik’s spots may form inside the mouth two or three days after the start of symptoms. A red, flat rash which usually starts on the face and then spreads to the rest of the body typically begins three to five days after the start of symptoms. Common complications include diarrhea (in 8% of cases), middle ear infection (7%), and pneumonia (6%).