Denying entry to people from Liberia, Guinea, and Sierra Leone would not stop the spread of Ebola into the United States, said Aditya Bhattacharji, an analyst at the Eurasia Group, a political risk firm. A ban, he noted, would be impossible to accomplish with 100 percent effectiveness.

What would happen with United States citizens visiting those countries and returning home, for instance? How about dual-nationals who don’t need a visa into the United States? How about other foreign nationals who visited West Africa?

Health quarantines have a long history, going back to the bubonic plague in Venice, said Dr. Howard Markel, a professor of pediatrics and communicable diseases at the University of Michigan. But in recent decades, even with diseases that are much more readily contagious than Ebola, travel bans have been rejected.

A travel ban was never adopted in the 2003 SARS outbreak, which started in Asia, although it affected about 8,000 people worldwide, killing 774 of them. In that case, the World Health Organization issued a travel advisory, Dr. Markel said.

The closest that the United States has come to a travel ban was barring entry to people with H.I.V. or AIDS in a 1987 decision during the Reagan administration. That ban, which did not apply to specific countries, was lifted by President Obama.

Mr. Bhattacharji pointed out that after the terror attacks of Sept. 11, 2001, all flights in the United States were grounded for days. That radical measure had an unexpected consequence: it delayed the flu season by two weeks. It was the only example he could point to where travel restrictions actually slowed down the spread of a disease.

While the prevention and screening measures in place today are far from perfect, they allow public officials to track down sick travelers and understand their travel pattern. A ban might lead people to hide their tracks, making it harder to find out who they met or where they came from.