It is extremely unlikely to spread through public transit, for several reasons.
Not all viruses build up to infectious doses in all bodily fluids. For example, measles is coughed out because it first invades cells at the back of the throat, while H.I.V. is not. Norovirus is not a respiratory virus, but such small doses are needed to infect a person that aerosolized vomitus is thought to have sickened many cruise ship passengers. Normally, Ebola does not at first make victims cough or sneeze, although someone who also had the flu could, in theory, spray vomitus or blood. Once Ebola invades the lungs, the body will cough to clear them. But passengers that deathly ill are not likely to be on public transit.
According to the recent W.H.O. statement, high levels of Ebola virus in saliva are rare except in the sickest victims, and whole virus has never been found in sweat. The fluids known to build up high viral loads are blood, feces and vomit.
How much virus is needed to cause illness is not exactly known. Viruses differ that way. In any group that shares needles, hepatitis C will spread more readily than H.I.V. because smaller doses infect.
One tantalizing possibility is that very small doses of Ebola act as a vaccine. Scientists working in Gabon have found that more than 30 percent of the populations of some villages have Ebola antibodies, although they have never been sick or in contact with anyone who was. They may have swallowed some virus by eating infected bats or fruit contaminated with bat saliva. (Alternatively, said W. Ian Lipkin, a virus expert, they may have had an unknown virus that cross-reacted in antibody tests.)
No one has tested Ebola transmission on subways. But no case of transmission to a human from a dry surface has ever been confirmed. The C.D.C. has said there is “no epidemiological evidence” for transmission from hospital surfaces, including bed rails and door knobs – which are as close as a hospital room gets to having a subway pole and a bus handle. A 2007 study cited by C.D.C. experts shows that swabs of 31 surfaces — including bed frames, a spit bowl and a used stethoscope — in a very dangerous environment, an active Ebola ward in Uganda, — did not have virus in a single sample.
So how might Ebola be passed on a subway? 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.
Africa is full of overcrowded public transport — buses, minivans and some trains. There are no known instances of transmission in those environments.
And update tonight:On July 20, a dying Liberian-American flew to Nigeria and was vomiting on the plane. All 200 people aboard were monitored; none fell ill.
Can you get Ebola from a bowling ball?
A. Although the surface of a shared bowling ball is a likely place to find germs — and some people avoid bowling for this very reason — it is extremely unlikely that Ebola could be passed that way.
There is no evidence that it has been passed, as colds or flu sometimes are, by touching surfaces that someone else touched after sneezing into their hand. Ebola is normally passed through contact with blood, vomit or diarrhea.
If someone left blood, vomit or feces on a bowling ball, and the next person to touch it did not even notice, and then put his fingers into his eyes, nose or mouth, it might be possible. But, the Ebola virus does not not normally build up to high levels in saliva or mucus until very late in the disease — several days after the initial fever sets in — and it is unlikely that someone that ill would have just gone bowling. Also, the Ebola virus is fragile and susceptible to drying out. It does not normally survive for more than a few hours on a hard, dry surface.