Can you catch Ebola on a crowded bus or train if you are standing next to someone who is infected? What if that person sneezes or coughs on you? If the person has symptoms, the answer could be yes.

Questions that may have seemed theoretical a few days ago have taken on a troubling reality since a traveler from Liberia, Thomas E. Duncan, carried the virus to Texas and potentially exposed dozens of people before he was placed in isolation. And the hospital where he first sought help failed to make the diagnosis, leaving him in the community for several days when he was becoming more and more contagious.

It could happen again, said Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention, in a telephone news briefing on Thursday. He said there was no way to detect the disease during the incubation period — the interval before symptoms set in — so other infected people could pass fever checks at airports in West Africa, just as Mr. Duncan did, and board planes to the United States.