The Centers for Disease Control and Prevention confirmed Sunday that a nurse at a Dallas hospital who cared for Thomas Eric Duncan, who died from Ebola last week, was the first person to become infected with the virus on U. S. soil. The nurse reportedly wore a gown, gloves, a mask and a face shield while caring for the Liberian national at Texas Health Presbyterian Hospital. Many, including CDC Director Tom Frieden, are questioning how the nurse became infected despite wearing the appropriate personal protective equipment, which should have shielded her from direct contact with Duncan and his bodily fluids.

Once again, the specter of airborne Ebola is being raised.

No virus that causes disease in humans has ever been known to mutate to change its mode of transmission. This means it is highly unlikely that Ebola has mutated to become airborne. It is, however, droplet-borne — and the distinction between the two is crucial.

Doctors mean something different from the public when they talk about a disease being airborne. To them, it means that the disease-causing germs are so small they can live dry, floating in the air for extended periods, thus capable of traveling from person to person at a distance. When inhaled, airborne germs make their way deep into the lungs.

Chickenpox, measles and tuberculosis are airborne diseases. Droplets of mucus and other secretions from the nose, mouth and respiratory tract transmit other diseases, including influenza and smallpox.

When someone coughs, sneezes or, in the case of Ebola, vomits, he releases a spray of secretions into the air. This makes the infection droplet-borne. Some hospital procedures, like placing a breathing tube down a patient’s air passage to help him breathe, may do the same thing.

Droplet-borne germs can travel in these secretions to infect someone a few feet away, often through the eyes, nose or mouth. This may not seem like an important difference, but it has a big impact on how easily a germ spreads. Airborne diseases are far more transmittable than droplet-borne ones.

Richard Preston‘s remarkable book, The Hot Zone, chronicled an Ebola Reston virus outbreak at a primate quarantine facility just outside Washington. The monkeys didn’t have direct contact with each other. CDC and military experts had to consider the possibility that Ebola Reston virus might be airborne. But feces thrown about the room, aerosols used in pressure washing the monkey cages or contaminated gloves used to handle the animals could also have transmitted the virus.

It is important to emphasize that Ebola Reston virus does not cause disease in humans. It also survives longer than Ebola Zaire—the species responsible for the West African epidemic—when aerosolized in the lab. But even Ebola Zaire can remain infectious when aerosolized for at least 90 minutes. What happens in the lab, however, doesn’t always represent the real world.

Since the Reston scare, scientists have learned more about Ebola transmission from other outbreaks.

In 1995, more than 300 people became sick with Ebola in Kikwit, Democratic Republic of the Congo. Disease detectives were unable to determine how 12 of the patients were exposed — again raising questions about the possibility of airborne transmission. But if Ebola could be transmitted through the air, at least some family members of Ebola patients should have gotten sick even without direct contact. That didn’t happen.

Ebola struck again in 2000, this time affecting more than 400 people in Gulu, Uganda. Not all had direct contact with another Ebola patient. Bedding and mattresses seemed to be one source of infection. So did sharing a meal with an Ebola patient — which often meant using fingers to eat from the same plate. Each had in common likely exposure to infected bodily fluids.

In the lab, scientists studied how Ebola virus infects different species and causes disease. In humans and primates, Ebola Zaire spreads from the cells of the immune system to the lymph nodes, blood, liver and spleen. It causes minimal disease in the lungs. But in pigs, Ebola Zaire causes severe lung disease.

Researchers infected pigs with Ebola Zaire and then placed them near but not in direct contact with primates. The primates became infected. Because Ebola Zaire causes severe lung disease in pigs, their respiratory secretions are laden with the virus. With all their snorting and snuffling, pigs are very good at generating aerosols. The infected monkeys, however, didn’t transmit the virus onward.

For Ebola Zaire to become airborne in humans, it would need to cause lung disease significant enough to release lots of virus into respiratory secretions. The virus would then need to survive outside the body, dried and in sunlight for a prolonged time. And it would need to be able to infect another person more than a couple feet away.

There’s no evidence from previous epidemics or laboratory experiments that Ebola Zaire behaves in this way. Although the virus is mutating as the Ebola epidemic continues to grow in West Africa, it has multiple hurdles to overcome in order to become airborne.

As we rule out Ebola being airborne, the droplet-borne risk of Ebola must be addressed. Most important, those on the frontlines—especially nurses and doctors—should be provided with the necessary training and personal protective equipment to ensure that there are no more transmissions within hospitals.

PHOTO: A member of the CG Environmental HazMat team disinfects the entrance to the residence of a health worker at the Texas Health Presbyterian Hospital who has contracted Ebola in Dallas, Texas, October 12, 2014.REUTERS/Jaime R. Carrero