No, seriously—how contagious is Ebola?"The reproduction number, or "R nought," is a mathematical term that tells you how contagious an infectious disease is. Specifically, it’s the number of people who catch the disease from one sick person, on average, in an outbreak.

"Take, for example, measles. The virus is one of the most contagious diseases known to man. It’s R0 sits around 18. That means each person with the measles spreads it to 18 people, on average, when nobody is vaccinated. (When everyone is vaccinated, the R0 drops to essentially zero for measles).

”And that brings us back to Ebola. Despite its nasty reputation, the virus’s R0 really isn’t that impressive. It typically sits around 1.5 to 2.0.”