How infectious is Ebola?

The US case of Ebola is the first to be identified outside of Africa. The patient, who has now died from the disease, didn’t know that he was infected straight away, so wasn’t quarantined by the Texas Health Presbyterian Hospital in Dallas for four days. This sounds like a long time to be walking around being contagious, but medical authorities have assured the public not to panic.

In fact, officials seem pretty sure that this particular case would not lead to an outbreak in the US. "I have no doubt that we will control this importation, or case, of Ebola so that it does not spread widely in this country," director of the Centers for Disease Control and Prevention (CDC) Tom Frieden told the press earlier this week

Professor of integrative biology Lauren Ancel Meyers from the University of Texas agreed, telling The Huffington Post science editor David Freeman, "I think they are striking a good note in saying that most of you out there don’t have to worry. There doesn’t seem to be a real threat of a large epidemic in the United States."

The reason for Frieden’s and Meyers's confidence is a simple mathematical term known as R0. This is a 'reproduction number' used by epidemiologists to indicate how infectious a specific disease is. It tells you how many people, on average, will be infected by one patient. 

"The reproduction number provides a lot of information," Meyers told The Huffington Post. "It gives us a baseline for projecting the growth of outbreaks in the absence of intervention, and it tells us how hard and how effective do our interventions have to be in order to stop an epidemic."

Measles, for example, is the most contagious disease we have, and its R0 is about 18. This means that if no one is vaccinated, an incredible 18 people can be infected by every one person who has the disease. Of course, this number drops to zero if everyone is vaccinated. For HIV/AIDS and SARS, the R0 number is between 2 and 5, and for Ebola, it’s just 2. 

According to Michaeleen Doucleff at NPR, while many factors influence a disease’s reproduction number, the fact that Ebola’s is transmitted via bodily fluids, rather than the air, is probably why it’s rated so low. And because Ebola isn't contagious until the patient starts showing symptoms - at which point the Dallas patient had checked himself in to the hospital - all that needed to be done to contain the spread of the disease in the US is to isolate anyone at the hospital who might have been infected.

CNBC reports that as of yesterday, none of the 48 people who potentially came in contact with Dallas patient have developed any definite symptoms.

"Then R0 drops to zero. And Texas is free of Ebola,” says Doucleff. 

Here's how medical staff figure out who might have become infected by the Dallas patient: