Why Measles is So Contagious

In 1846, before we even knew what a virus was, doctor Peter Panum observed a measles outbreak in the Faroe Islands.

Because the Faroe Islands were so remote, Panum had an easy time observing the disease spread from person to person. He developed an eerie power of prediction. If one person developed a rash from measles, Panum knew that everyone else in the patient’s house would get sick two weeks later.
Panum noticed other predictable patterns, too. On average, he estimated, every infected person infected seven to nine other people. Today, the estimate for the average number of infections spread from each sick person is higher–between 12 and 18. By comparison, the figure for Ebola is only about two.

The panic over the Ebola virus is understandable because the mortality rate is high, but the chances of catching it are tiny compared to measles. The measles virus evolved to be very contagious in order to survive as a species. The virus must find new human hosts because old hosts become immune. Carl Zimmer explains the process in much more detail at The Loom.