Worried about Ebola? Take a deep breath. This is a terrible crisis in West Africa, but it is exceedingly unlikely that it will become a medical crisis here in the United States. Dallas has seen one tragic case. So far there have been no confirmed additional cases (cross your fingers).

Our story today on the math of Ebola is certainly disquieting. We focused on the ghastly number of infections and deaths in West Africa. Our story makes clear that the virus is still outpacing the response.

Right now, the math still favors the virus.

Global health officials are looking closely at the “reproduction number,” which estimates how many people, on average, will catch the virus from each person stricken with Ebola. The epidemic will begin to decline when that number falls below one. A recent analysis estimated the number at 1.5 to 2.

The number of Ebola cases in West Africa has been doubling about every three weeks. There is little evidence so far that the epidemic is losing momentum.

There are some hopeful signs — seven provinces in Guinea where new infections haven’t been seen in three weeks, for example. People are changing burial practices and there are more safe burials now. But the big picture is the same, which is that there is a huge gap between what is needed (for example, hospital beds, doctors, nurses) and what is available.

What you shouldn’t do it let people persuade you that the virus is going to run rampant all over the planet. Some of the comments on the Ebola math story skew toward Doomsday scenarios. For example, one reader took the worst-case CDC scenario (one that presumed a straight extrapolation of recent infections and not effective response) and kept extrapolating, and determined that, “left unchallenged,” Ebola cases by next September “would exceed the population of the planet.” Let’s assume that’s a discussion point and not a prediction. (Hmm, maybe the reader means that the virus will spread to other inhabited planets?)

Another person wrote:

“This is much worse than aids. It is much much more contagious. You could touch a person with Aids. You don’t want to breath the same air as an ebola.”

Wrong. This is not an airborne virus. Ebola is not nearly as contagious as many other diseases. (See my recent To Your Health blog post that explains the difference between “infectious” and “contagious.”) “Hot” viruses such as Ebola are contagious only when the victim has symptoms and is shedding virus in bodily fluids.

But wait: Isn’t there evidence that Ebola can be spread through “aerosol” transmission?

You may have stumbled across material on the Internet, in reputable journals, indicating just that. For example, there’s this study in the International Journal of Experimental Pathology, titled “Lethal experimental infections of rhesus monkeys by aerosolized Ebola virus.” The abstract concludes:

Demonstration of fatal aerosol transmission of this virus in monkeys reinforces the importance of taking appropriate precautions to prevent its potential aerosol transmission to humans.

This, however, is a laboratory experiment that does not in any way mimic what the virus does in the natural world. So says one of the co-authors, Peter Jahrling, a scientist who works with Ebola in a laboratory in Frederick, Md., that’s part of NIH’s National Institute of Allergy and Infectious  Diseases.

“The aerosol studies were done to inform the community about the threat of a truly weaponized threat delivered by a competent adversary. Those conditions should not be extrapolated to a natural disease setting,” Jahrling told me.

One story published this week, in the Los Angeles Times, raised the possibility that Ebola could be spread through the air via sneezing. Scientists certainly can’t rule that out, a CDC spokesman told the Times. Another scientist suggested that monkeys who came down with Ebola in Reston in 1989 may have spread it through sneezing. But this is not a true “airborne” contagion. A sneeze creates small droplets, not aerosol particles that can float through the air and perhaps move through a ventilation system, Jahrling said.

The World Health Organization directly addresses this:

Theoretically, wet and bigger droplets from a heavily infected individual, who has respiratory symptoms caused by other conditions or who vomits violently, could transmit the virus – over a short distance – to another nearby person.

This could happen when virus-laden heavy droplets are directly propelled, by coughing or sneezing (which does not mean airborne transmission) onto the mucus membranes or skin with cuts or abrasions of another person.

WHO is not aware of any studies that actually document this mode of transmission. On the contrary, good quality studies from previous Ebola outbreaks show that all cases were infected by direct close contact with symptomatic patients.

Okay, but could the virus mutate to become airborne? Hasn’t even Tom Frieden, who has tried to tamp down Ebola hysteria, admitted that it’s not impossible for Ebola to mutate to become more contagious?

The short answer is that there’s zero evidence this has happened with this virus, it doesn’t seem to mutate very quickly, and although you can’t absolutely rule out the possibility, such a thing has never happened before in recorded history.

Thus Jahrling says: “The likelihood is remote squared.”

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The terrifying logic of Ebola math

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