Health officials at London's Heathrow Airport have begun screening travellers from West Africa for Ebola. 

The current outbreak of the virus is the worst on record and has already killed more than 4,000 people -- mostly in Liberia, Sierra Leone and Guinea.

Some have raised doubts about the effectiveness of border screening, since an infected person may not immediately show symptoms.

But Paul Cosford, who's the director of Public Health England, says he believes it will help. 

"Obviously, this is a very new process, we've only started it today. We'll be learning any possible lessons from people who have not managed to go through the process if that's the case. But we are confident that the system is robust and in place as of this morning."

Ebola screenings -- which may include questionnaires and temperature checks -- will be extended to London's Gatwick airport and the Eurostar rail link to Europe by the end of next week.