When is a person with measles contagious?

A person with measles is contagious from approximately 4 days before the rash appears through 4 days after rash onset — a total infectious window of about 8 days. This means an infected person is spreading measles before they even know they have it, since the rash does not appear until the disease has been progressing for several days. During the prodromal phase (before the rash), the only symptoms are fever, cough, runny nose, and red eyes — easily mistaken for a common cold or flu. This pre-rash contagiousness is one of the main reasons measles outbreaks are so hard to contain: by the time someone is diagnosed, they may have already exposed many others. People with measles should be isolated from others from 4 days before rash onset through 4 days after rash onset, per CDC guidelines.