A scary problem lurks beyond the frenzied efforts to keep people from spreading ebola: No one knows exactly where the virus comes from or how to stop it from seeding new outbreaks.

Tens of thousands of letters mailed around the US could have picked up trace amounts of anthrax while passing through a contaminated postal centre, the nation’s top anthrax investigator has said.

Federal officials said today they believed an anthrax-filled letter that has yet to be found sickened a State Department mail handler - a theory bolstered by traces of anthrax in eight spots in the Washington building where he worked.

Health workers in the US are being vaccinated against smallpox, amid fears the virus may be used in a terrorist attack.