DAKAR, Senegal (AP) — 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.