Measles

Measles is a highly contagious disease caused by the measles virus, an RNA virus of the paramyxovirus family. It spreads through the air and is among the most infectious human diseases, with each case infecting 12 to 18 unvaccinated people on average, a value of the basic reproduction number far higher than most other pathogens.

After an incubation period of about ten days, measles causes high fever, cough, runny nose and red eyes. Small white spots called Koplik spots may appear inside the mouth, followed by a red rash that spreads from the face downward. Complications include pneumonia, the most common cause of death, and brain inflammation. The virus also damages immune memory, which leaves children more open to other infections for months or years afterwards.

Two doses of measles-containing vaccine, usually given as MMR, are about 97 percent effective. Because the virus is so contagious, around 95 percent of a population must be immune to stop its spread, a level of herd immunity higher than for most diseases. Measles was declared eliminated in the United States in 2000, and falling vaccination coverage has since allowed outbreaks to return in many countries.

The World Health Organization estimates that measles still causes over 100,000 deaths a year, mostly among unvaccinated children under five. Tracking cases and vaccination coverage is a core task of epidemiology and public health, and measles outbreaks are often the first sign that immunization programs have fallen short.