Liberia has announced the quarantine of a number of communities and the closure of schools across the country, the toughest measures yet imposed by a West African government in a bid to halt the worst Ebola outbreak on record.

No one knows 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.

A US doctor working with Ebola patients in Liberia has tested positive for the deadly virus.