Mayor Bill de Blasio rides the subway in New York this morning. Courtesy of the NYC Mayor's Office .

To prove the point that New York City's subway isn't infected with Ebola, Mayor Bill de Blasio did what a politician must: He rode it himself this morning.

Commuters are understandably jittery after news broke Thursday night that the city now has its first case of Ebola. Craig Spencer, the doctor who contracted the disease while volunteering in Guinea, rode several lines of the MTA — the L, A and 1 trains — the day before his diagnosis (he also took an Uber Wednesday night, although the threat of contact there is clearly more limited).

These two facts — a New Yorker has Ebola, and he rode the subway — conjure worst-case scenarios of how any contagious disease might quickly spread in the densest city in the United States. De Blasio, repeatedly photographed with a smile on his face and a cup of coffee in hand, is right, though, that there's not that much to fear here. While transportation generally, and mass transit in particular, have been crucial to the spread of diseases, Ebola probably won't be one of them.

It is not, for instance, influenza. Research has found that riding transit in flu season can increase your risk of catching an acute respiratory infection as much as six-fold. Similar results have been found linking certain bus routes in Houston to higher incidences of tuberculosis. The poor ventilation and close contact of minibuses are also thought to significantly contribute to the spread of tuberculosis in South African cities. Historically, public transportation was also a central public health concern during the spread of epidemics of influenza.

Both influenza and tuberculosis, though, can be transmitted by airborne particles — by one commuter coughing or breathing on another. Ebola is much more stubborn in how it spreads. You've probably read by now (if you live in New York, in comforting emails from your elected officials) that Ebola can't be spread by an infected person before he or she shows symptoms of the disease. Spencer reportedly developed his symptoms after riding the train. And the symptoms that we're talking about — high fever, vomiting, diarrhea — would probably prevent most of us from riding public transit in the first place.

The next big factor is that you have to come in contact with the bodily fluid of someone who has developed symptoms. There are no known cases of Ebola transmission through dry surfaces, meaning you're probably not going to catch it from sharing a hand rail on the train or sitting on a seat previously occupied by someone who's sick.

Yes, there is an out-there scenario where you catch Ebola on mass transit. Emphasis on out-there. This is what would have to happen, according to a good but graphic explainer from the New York Times:

If someone ejected bloody mucus or vomitus onto a subway pole, and the next passenger were to touch it while it was still wet and then, for some unimaginable reason, were to put those wet fingers into an eye or mouth instead of wiping them in disgust — then yes, it could happen.  Similarly, if an extremely ill passenger with high viral saliva loads were to sneeze large, wet droplets directly into the mouth or eyes of another passenger, the infection might be passed. But the influenza route — sneeze to hand to pole to hand to eye — has never been known to happen and is considered extremely unlikely.

(The bold is mine.) Of course, it's easy to tell people in New York, from Washington, that their subway is perfectly safe (at least from Ebola). But, as with many other aspects of this particular disease, this is no doubt true of public transit, too: Fear will spread faster there than Ebola itself.