The last time that health officials tried to screen passengers, they met with mixed results, Dr. Bloom said. Confronted with the SARS epidemic in 2003, some countries sought to screen passengers, but in the experience of Australia, Canada and Singapore, these screenings failed to turn up a single SARS case.

At least 14 Ebola cases have been treated outside West Africa in the current outbreak. Most of those involve health and aid workers who contracted the disease in West Africa and were flown back to their home countries for medical treatment.

The virus has started to raise concerns among those who work on airlines. On Tuesday, union representatives of flight attendants and airport ground workers called for tougher screening. In addition, the United States Coast Guard says it is planning to increase screening procedures for passengers aboard cargo ships coming into American ports from West Africa.

Two people outside West Africa have been found to have the virus. One is a Liberian man who began showing symptoms four days after arriving in Dallas and the other a Spanish nurse who became ill after treating a missionary in a hospital in Madrid.

But screening passengers at their point of departure has flaws since it relies on them to tell the truth. In the one known Ebola case in the United States, the man infected with the virus had lied about not having any contact with an Ebola patient, the authorities said.

European countries face the same dilemma. The health authorities in Spain have quarantined three people and are monitoring dozens of others who came into contact with the nurse, the first known case of the disease in Europe. In Britain, the health authorities have said there is a real risk that the virus will be imported but have acknowledged that they have no plan to screen visitors entering the country for Ebola.