Here's how much more contagious measles is than EbolaMeasles, a disease we thought was history, is making a worrisome comeback. Most of the reported cases have been in California , where dense clusters of children are going without the vaccine because their parents have refused it. These clusters are particularly dangerous because they threaten herd immunity, the scientifically-proven concept that so long as a certain percentage of people in a community are vaccinated, the majority is protected from a disease (including those who are too young or too ill to be vaccinated). As of the late 1990s, measles was a disease that had all but disappeared in the United States. This month, thanks in part to a recent flare-up at Disneyland, the number of reported cases shot up to 102, a record for a disease that’s reached 100 annual cases only a handful of times since 2001. But measles, viral infection of the respiratory system , immune system , and skin, spreads quickly — so fast that it’s roughly 9 times more contagious than Ebola. For every person who has measles, about 18 more will get it. Part of the reason it’s so infectious is that it’s airborne, meaning that unlike Ebola (which spreads only via the bodily fluids of infected people), it can be spread via a stray cough or sneeze and survive for up to two hours on surfaces and in the air. Also unlike Ebola, someone with measles doesn’t have to have symptoms to spread it. Once you get measles, you can infect others for four days before you develop any visible symptoms, such as the telltale all-over-the-body rash or a hacking cough. Obviously, those who haven’t gotten the measles vaccine are at the highest risk of infection. For those people, roughly 90% of those exposed to the virus will get sick. Conversely, people who have received both doses of the MMR vaccine have only about a 1% chance of being infected when exposed. More