DAKAR, Senegal — No one knows for sure just how many people Patrick Sawyer came into contact with the day he boarded a flight in Liberia, had a stopover in Ghana, changed planes in Togo and then arrived in Nigeria, where authorities say he died days later from Ebola, one of the deadliest diseases known to man.

Now health workers are scrambling to trace those who may have been exposed to Sawyer across West Africa, including flight attendants and fellow passengers.

Health experts say it is unlikely he could have infected others with the virus that can cause victims to bleed from the eyes, mouth and ears. Still, unsettling questions remain: How could a man whose sister recently died from Ebola manage to board a plane leaving the country? And worse: Could Ebola become the latest disease to be spread by international air travel?

Sawyer’s death on Friday has led to tighter screening of airline passengers in West Africa, where an unprecedented outbreak that emerged in March has killed more than 670 people in Guinea, Sierra Leone and Liberia. But some health authorities expressed little confidence in such precautions.

“The best thing would be if people did not travel when they were sick, but the problem is people won’t say when they’re sick. They will lie in order to travel, so it is doubtful travel recommendations would have a big impact,” said Dr. David Heymann at the London School of Hygiene and Tropical Medicine.

“The important thing is for countries to be prepared when they get patients infected with Ebola, that they are isolated, family members are told what to do, and health workers take the right steps.”

The World Health Organization is not recommending any travel restrictions since the outbreak came to light.

The risk of travelers contracting Ebola is considered low because it requires direct contact with bodily fluids or secretions such as urine, blood, sweat or saliva, experts say. Ebola can’t be spread like flu through casual contact or breathing in the same air.

Patients are contagious only once the disease has progressed to the point they show symptoms, according to the WHO. And the most vulnerable are health care workers and relatives who come in much closer contact with the sick.

Still, witnesses say Sawyer, a 40-year-old Finance Ministry employee en route to a conference in Nigeria, was vomiting and had diarrhea aboard at least one of his flights with some 50 other passengers aboard. Ebola can be contracted from traces of feces or vomit, experts say.

Sawyer was quarantined upon arrival in Lagos and Nigerian authorities say his fellow travelers were advised of Ebola’s symptoms and then were allowed to leave. The incubation period can be as long as 21 days.

Health officials rely on “contact tracing” – locating anyone who may have been exposed, and then anyone who may have come into contact with that person. That may prove impossible, given that other passengers journeyed on to dozens of other cities.

Nigerian authorities so far have identified 59 people who came into contact with Sawyer and have tested 20, said Lagos State Health Commissioner Jide Idris. He said there have been no new cases of the disease.