Most contagious disease

The most contagious known disease is measles, a viral infection caused by the virus Measles morbillivirus. The basic reproduction number (R₀) for measles is between 12 and 18, meaning that a single infected person will on average spread the disease to around 15 other people in unvaccinated populations.

Measles is spread through aerosol or airborne transmission – a fine mist of water and virus particles that are expelled from the body by coughing, sneezing or just breathing. These fine droplets can be carried a considerable distance and spread through ventilation systems. Less contagious respiratory infections, such as influenza or the common cold, spread through what are called "respiratory droplets". These are larger drops of virus-laden water, which do not carry as far as aerosolized droplets. As a consequence these illnesses have a lower R₀ of around 1.5–3.

Measles was once a common childhood illness, but vaccinations effectively ended its transmission in most developed nations during the second half of the 20th century. In recent years, however, declining vaccination rates have seen it make a return in many countries. It was also never eliminated in large parts of the world, and remains a major problem in less developed nations.

The illness causes a high fever, cough and, most notably, a painful rash. Though most people who are infected escape with no lasting health issues, around 25% need to be hospitalized and 0.2% will die. Measles is responsible for around 140,000 deaths every year, mostly of children under 5.