Health officials in the UK say they are monitoring closely a deadly outbreak of swine flu in Mexico, amid fears of a potential pandemic.
The Health Protection Agency (HPA) said it was working with the government to assess any threat posed to the public.
It described the outbreak as "unusual" and warranting "further investigation and vigilance" by other countries.
Northwick Park Hospital said a BA worker who arrived from Mexico City and fell ill does not have swine flu.
At least 81 people in Mexico are now thought to have died after contracting a new strain of flu being linked to an outbreak of swine flu in the US.
There have been no confirmed cases so far in Europe.
Swine flu is a respiratory disease that infects pigs and does not normally infect humans.
However, sporadic cases do occur, usually in people who have had close contact with pigs.
The WHO said some of those affected in Mexico had tested positive for a strain - H1N1 - that had infected at least seven people in the south-western US.
The concern is that this strain has the potential to spread rapidly because it appears to be passing easily from human to human.
WHO Director-general Margaret Chan said the strain had "pandemic potential" but that it was too early to say whether one would actually occur.
Source: BBC News