Ebola is not like the flu. You're not going to catch the disease from simply being across the room from someone who has it.
Basically every health agency in the world agrees on this point. In order to catch Ebola, you have to come in contact with bodily fluids — such as sweat, saliva, vomit, or diarrhea — of someone who already has Ebola and is exhibiting symptoms. (A person who is infected but not yet showing symptoms generally isn't infectious.)
And yet, remarkably, many readers have been unconvinced by this point. They keep e-mailing and pointing to a 2012 paper finding that, in a single laboratory study, Ebola spread from pigs to monkeys that hadn't directly touched each other. Some readers claim this paper is proof that Ebola can travel miles through the air to kill people.
Those claims are wrong — but what does the study really say? To find out, I called up the first author of that study, Hana Weingartl, a scientist at the Canadian Food Inspection Agency.
Why pigs are different from people
Here's what actually happened in the 2012 experiment. Six piglets with Ebola were housed next to four monkeys separated in cages. A buffer zone of roughly 8 inches separated the pigs from the monkeys so that they couldn't touch each other directly. Then, two of the monkeys got Ebola fast enough that it was clear that they caught it from the pigs.
But just because this happened between pigs and monkeys doesn't mean it's likely to happen between people. The big difference is that pigs cough and sneeze a lot when they're sick with Ebola — way more than people do.
"You cannot take the pigs and think that it will go the same way in humans," Weingartl said. "One has to consider the species. For pigs, the [Ebola] infection ends up as an infection of the lungs — they have high amounts of the virus in the respiratory tract and so they cough it out. Or when they sneeze or squeal, it just gets out of the lungs. So the virus is in the air directly."
But Ebola affects primates in a different way, Weingartl says. For them, "the main target organ is the liver, so they have high amounts of the virus in the blood and in the feces. They will not be coughing out the virus. And that’s why indirect transmission without contact is probably not happening [among primates and humans]."
Several papers have addressed possible transmission between primates in laboratory settings, including one published on July 25. That one showed no airborne transmission. Another from back in 1995 described transmission without direct contact, but couldn't determine if this was from big droplets, tiny aerosol droplets, or something else.
Of course, it's entirely possible that a big spit droplet from a human Ebola patient could fly a few feet through the air and land on someone else. But current Ebola protection measures seem to guard against this. Health-care workers are told to cover their faces and bodies with protective gear, for example, and patients are generally separated from the general population by a buffer zone of plastic fencing.
It's highly unlikely that Ebola can travel long distances through the air
What some readers seem to be worried about, though, is not a big cough droplet that travels a few feet, but whether Ebola could travel longer distances in tiny, tiny droplets. This is called "aerosol transmission," and it's something that the measles and some kinds of influenza can do.
Experts say that this is highly unlikely. And this 2012 study of pigs and monkeys doesn't contradict that — indeed, it didn't really address the question of aerosol transmission. It couldn't distinguish between big droplets and little aerosol droplets because the pigs were simply too close to the monkeys.
What the study was designed to do is figure out if Ebola could go from a pig to a primate without them directly touching. Researchers were curious about this because there was evidence that a different, nonlethal-to-humans species of Ebola had done so in the Philippines.
So yes, two monkeys got Ebola from pigs in a laboratory. But that doesn't mean that they would have gotten it if they'd been 10 feet away from each other. And it doesn't mean that any of this would necessarily happen in people.
Further reading: Epidemiologist Tara Smith has several interesting posts on this topic (pig-to-monkey Ebola, are we sure Ebola isn't airborne, primate-to-primate study).