Why Is Measles So Contagious?

An infected person who coughs or sneezes can spray the virus into the air in small droplets, which can remain suspended for up to two hours. People with measles are infectious for about four days before and four days after the appearance of the rash. Before there was a vaccine, nearly everyone in the United States became infected with measles before the age of 15. The disease kills about 1 in every 1,000 people infected, and those who survive have lifelong immunity.