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.

SWINE FLU Swine flu is a respiratory disease found in pigs Human cases usually occur in those who have contact with pigs Human-to-human transmission is rare and such cases are closely monitored

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:

• Eleven confirmed infections in the US
• In addition, eight suspected cases are being investigated at a New York City high school where about 200 students fell mildly ill with flu-like symptoms
• Ten New Zealand students among a group which travelled to Mexico have tested positive for flu - "likely", though not definitely, swine flu, said Health Minister Tony Ryall
• But a UK hospital conducting tests for swine flu on a British Airways cabin crew member admitted on Saturday said the tests proved negative.

Mexico shutdown

In Mexico, public buildings have been closed and hundreds of public events suspended.

Schools in and around Mexico City have been closed until 6 May, and some 70% of bars and restaurants in the capital have been temporarily closed.

People are being strongly urged to avoid shaking hands, and the US embassy has advised visitors to the country to keep at least six feet (1.8m) from other people.

It's eerily quiet here in the capital. Lots of people with masks BBC reader Dr Duncan Wood, Mexico City

Mexico's Health Secretary, Jose Cordova, said a total of 1,324 people had been admitted to hospital with suspected symptoms since 13 April and were being tested for the virus.

"In that same period, 81 deaths were recorded probably linked to the virus but only in 20 cases we have the laboratory tests to confirm it," he said.

Mexico's President Felipe Calderon has announced emergency measures to deal with the situation.

They include powers to isolate individuals suspected of having the virus without fear of legal repercussions.

'International concern'

In the US, seven people in California, two people in Texas, and two people in Kansas have been infected with the new strain.

In New York, city health commissioner Dr Thomas Frieden said preliminary tests conducted on the ailing students showed they were possible cases of swine flu.

Further tests will clarify if it was the same strain that was detected in the other three states.

Following a meeting of its emergency committee on Saturday, the WHO said the virus had the potential to become a pandemic but it was too early to say whether that would happen.

PANDEMIC CRITERIA Virus can infect people It can easily spread person-to-person No immunity in humans

WHO Director General Margaret Chan said recent events constituted "a public health emergency of international concern" and that countries needed to co-operate in heightening surveillance.

The WHO is advising all countries to be vigilant for seasonally unusual flu or pneumonia-like symptoms among their populations - particularly among young healthy adults, a characteristic of past pandemics.

Officials said most of those killed so far in Mexico were young adults - rather than more vulnerable children and the elderly.

Source: BBC News