No, Seriously, How Contagious Is Ebola?
Holy moly! There’s a case of Ebola in the U.S.!
That first reaction was understandable. There’s no question the disease is scary. The World Health Organization now estimates that the virus has killed about 70 percent of people infected in West Africa.
The Ebola case in Dallas is the first one diagnosed outside Africa, the Centers for Disease Control and Prevention said Tuesday. And the health care system in Texas didn’t quarantine the man right away. He was sick with Ebola — and contagious — for four days before he was admitted to the hospital.
But when you look at health officials responding to the case in Dallas, they seem cool as cucumbers, despite the initial misstep.
"I have no doubt that we will control this importation, or case, of Ebola so that it does not spread widely in this country," said the director of the CDC, Dr. Tom Frieden.
Why is Frieden so sure this virus won’t spread beyond a handful of cases?
It boils down to something called “R0.”
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).
Image by Adam Cole/NPR