The Mexican authorities say 81 people are now thought to have been killed by an outbreak of a human swine flu virus.

People have been told to stay at home to contain the infection, which the World Health Organization (WHO) has warned could become a pandemic.

Confirmed or suspected cases have also been reported in the US and elsewhere.

At least some confirmed cases show a new version of the H1N1 swine flu sub-strain - a disease which infects pigs but only sporadically infects humans.

H1N1 is the same strain that causes seasonal flu outbreaks in humans, but the newly detected version contains genetic material from versions which usually affect pigs and birds.

There is currently no vaccine for the new strain but severe cases can be treated with antiviral medication.

It is unclear how effective currently available flu vaccines would be at offering protection against the new strain, as it is genetically distinct from other flu strains.

The respiratory virus is spread mainly through coughs and sneezes.

Although all of the deaths so far have been in Mexico, the flu is spreading in the United States and suspected cases have been detected elsewhere: