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How Do You Catch Ebola: By Air, Sweat Or Water?

A mock patient is wheeled in an isolation pod during a drill at the biocontainment unit in the Nebraska Medical Center in Omaha. Nati Harnik/AP hide caption

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Nati Harnik/AP

A mock patient is wheeled in an isolation pod during a drill at the biocontainment unit in the Nebraska Medical Center in Omaha.

Nati Harnik/AP

There's no question Ebola is one of the most terrifying diseases out there. It causes a painful death, typically kills more than 50 percent of those infected and essentially has no cure.

But if you compare how contagious the Ebola virus is to, say SARS or the measles, Ebola just doesn't stack up. In fact, the virus is harder to catch than the common cold.

That's because there has been no evidence that Ebola spreads between people through the air. Health experts repeatedly emphasize that human-to-human transmission requires direct contact with infected bodily fluids, including blood, vomit and feces.

And to infect, those fluids have to reach a break in the skin or the mucous membranes found around your eyes, mouth and nose.

But that hasn't stopped two-thirds of Americans from thinking that the virus spreads "easily," a poll from Harvard School of Public Health found in August. Almost 40 percent of the 1,025 people surveyed said they worry about an Ebola epidemic in the U.S. More than a quarter were concerned about catching the virus themselves.

Many questions still linger. Is Ebola really not airborne? Can it spread through contaminated water? What about through a drop of blood left behind on a table?

So we took those questions to two virologists: Alan Schmaljohn at the University of Maryland School of Medicine, and Jean-Paul Gonzalez at Metabiota, a company that tracks global infectious diseases.

Is the Ebola virus really not airborne?

With airborne illnesses, like influenza or tuberculosis, you can easily get sick by inhaling tiny pathogenic particles floating around in the air. But with Ebola, large droplets — which neither travel very far nor hang in the air for long — are the real risk factors.

That means an Ebola-infected person would likely have to cough or sneeze up blood or other bodily fluids directly in your face for you to catch the virus, Schmaljohn says. If that drop of blood doesn't land on your face, it will just fall to the ground. It won't be swimming in the air, waiting to be breathed in by an unsuspecting passerby.

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During the 1995 Ebola outbreak in the Democratic Republic of Congo, the U.S. Centers for Disease Control and Prevention followed 173 family members of 27 people infected with the virus. Seventy-eight people had no direct contact with an infected person, but they could have been exposed to Ebola through the air. None got infected.

The 28 family members who did get sick all had some sort of physical contact with an ill person.

"We've known for years that Ebola can be transmitted through direct contact with infected bodily fluids," Gonzalez says. "This is very clear, only direct contact."

So why does the question of airborne transmission keep coming up?

Well, "airborne" is a broad term that simply means "transported by air." If a drop of infected blood is sprayed directly from a needle to a person, then yes that contagious droplet was technically carried through the air.

But there is "no convincing epidemiological evidence that airborne transmission occurs from an infected person to a nearby non-infected person," Schmaljohn says.

The confusion surrounding this topic, he adds, is well-deserved, in part because health officials often try to simplify their messages. So they don't spend much time fleshing out "exceptions and extraordinary possibilities," he says.

"As they simplify it, they start to sound like an absolute," Schmaljohn says. "And people are smart enough to disbelieve absolutes."

He says to think about the methods of Ebola transmission as high-risk or low-risk. There's a high risk, for example, of catching Ebola if you shake hands with an infected patient and a very low one if that same person coughs or sneezes around you.

[Note: A study in 2012 showed infected pigs passed on the Ebola to monkeys without touching them. Besides the fact that people aren't pigs or monkeys, the lead author has pointed out it's possible the monkeys caught the virus when droplets from the pigs splashed into their cage during cleaning.

The authors also noted that pigs can generate large infectious droplets better than any other animal. And airborne transmission of Ebola between monkeys has never been observed.]

Although it's clear that Ebola is passed through bodily fluids, are some fluids more effective transmitters than others?

The virus is most abundant in blood and diarrhea. In fact, a milliliter of blood typically carries about a million infectious particles. And in a controlled lab, just a small drop of blood from an infected monkey can be strong enough to kill a million of its companions, Schmaljohn says.

A CDC study in 2007 found that Ebola is shed through other bodily fluids during the illness, such as saliva, breast milk and semen. In most cases, these fluids were not visibly contaminated by blood, but they still contained the virus. That study didn't look at sweat, but another one suggested that Ebola could be passed on through sweat.

Researchers suspect the amount of Ebola in these other fluids, like saliva and sweat, to be much lower.

How long can Ebola virus particles survive in a drop of blood on a surface outside the body?

A drop of blood can remain contagious outside the body. And virus particles can survive for days or weeks, depending on the environment. Ultraviolet light, heat and exposure to oxygen gradually deactivate the virus, while cooler temperatures and humidity help keep it active.

"Blood, once it's outside the body, contains all the virus it's ever going to contain," Schmaljohn adds. "It's all downhill from there [for Ebola]."

A burial team in Barkedu, Liberia, buries their protective clothing alongside the body of an Ebola victim. It's possible to catch the virus from clothing soiled by infected blood or other bodily fluids. Tommy Trenchard for NPR hide caption

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Tommy Trenchard for NPR

A burial team in Barkedu, Liberia, buries their protective clothing alongside the body of an Ebola victim. It's possible to catch the virus from clothing soiled by infected blood or other bodily fluids.

Tommy Trenchard for NPR

So yes, there's a risk you can catch Ebola from a drop of blood on a table. But that risk diminishes over time as the blood dries up. Still, he stresses, surface decontamination is necessary in practice.

Can Ebola be spread through a drop of water or carried through the water system?

"[The virus] will not remain for a long time in the water," Gonzalez says. "It's not a very rich medium to protect the virus."

It's important, he adds, to remember that viruses aren't as resistant outside the body as bacteria are. Rather, they depend heavily on the cells of their host — animal or human — for survival.

In water, the Ebola virus would be deactivated in a matter of minutes, Schmaljohn says. That's because each Ebola virus is encased in an envelope taken from the outer surface, or membrane, of a host cell.

So what about cells in water that are infected with Ebola? Could you get the virus from infected cells in contaminated water?

Infected cells don't live long inside a liquid that doesn't have the same salt concentration as in our bodily fluids.

Drinking water has a lower salt concentration than that found inside human cells. As water rushes inside the cell to balance the salt concentration, pressure builds ups. Eventually the cell bursts, killing the virus in the process.

How long does an infected corpse remain contagious?

A long time — although it's still unknown exactly how long.

Remember, virus particles can last for days and even weeks in a drop of blood. So inside the entire body of the deceased patient, the virus can probably remain active for several months, Gonzalez says.

"That's why it's very important to [perform] the burial as soon as possible," he adds. And to be extremely careful while doing it.