Commuters on a subway train in New York late last month. (AFP/Getty Images)

At first glance, it’s a nightmare scenario: A New York City doctor who traveled to Guinea to fight what he called “one of the worst public health and humanitarian disasters in recent history” got infected with Ebola and returned to one of the nation’s most populous cities. Then, just as he was beginning to feel “sluggish,” he rode a subway system that carried a record 149 million passengers in September.

But it’s not as bad as it seems. And here’s why.

If there’s anyone who understands Ebola — who has studied the virus both under a microscope and in Ebola-ravaged patients — it’s Peter Piot. As a 27-year-old researcher, he discovered the disease in 1976 and today leads the renowned London School of Hygiene and Tropical Medicine. He has a bit of advice: Don’t panic about the subway. He wouldn’t.

Cases of Ebola arriving in the United States or Europe, he told Agence France-Presse in late July, won’t “give rise to a major epidemic. Spreading in the population here, I’m not that worried about it.” He added: “I wouldn’t be worried to sit next to someone with Ebola virus on the tube as long as they don’t vomit on you or something. This is an infection that requires very close contact.”

According to health officials, Craig Spencer, the infected doctor who traveled the A,L and 1 subway lines, was not yet symptomatic or feverish while in transit on Wednesday. This is very important, as Piot mentioned. For Ebola is highly infectious, but difficult to catch, even in an enclosed environment like a train or plane.

“There is no reason for New Yorkers to be alarmed,” New York Mayor Bill de Blasio (D) told reporters at Thursday night’s news conference. “Ebola is an extremely hard disease to contract. … Being on the same subway car or living near a person with Ebola does not in itself put someone at risk.”

Here's how the virus spreads and how contact tracing works to stop outbreaks. (Gillian Brockell/The Washington Post)

As explained by Vox, there are only a few ways you can get Ebola on a plane. You could come into “direct contact” with an infected person’s blood, feces, vomit, urine, semen, breast milk, saliva or tears. For that, you would need to share food, kiss or have sex with an infected person, getting body fluids on your hands and then touching your mouth, nose or eyes. Bottom line: not that easy.

Another way you could get Ebola, reported Vox, is to come into contact with an infected surface like a handlebar or door, and then touch one of your mucous membranes with that hand. According to the Centers for Disease Control and Prevention, household bleaches can easily obliterate the virus. But if it’s left unattended, “Ebola on dry surfaces, such as doorknobs and counter-tops, can survive for several hours” and “virus in body fluids (such as blood) can survive up to several days at room temperature.” But again, the virus can only be transmitted in this way by a patient who is already symptomatic.

Ebola is much different than history’s other scourges. Measles at one time spawned 17 secondary cases every time there was a new infection. Or the Spanish flu of 1918 produced two to five additional cases, with only two or three days separating the first and secondary cases. “In other words, it doesn’t take long for [that] flu virus to settle into a new host and a substantial number of transmissions can occur even before a person realizes that he or she has the flu,” according to a Washington Post opinion article.

But the best evidence we have Thursday evening’s news isn’t as bad as it first appears: Dallas. More than 21 days — the virus’s maximum incubation period — have already elapsed since Liberian Ebola victim Thomas Eric Duncan arrived in the United States. And with the exception of two nurses, no new cases of Ebola emerged in anyone who had contact with Duncan in a non-hospital setting. Not his family. Not his friends.

And not anyone aboard the plane he rode to the United States.