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The World Health was on the alert Saturday, saying "very concerned" about the appearance of a swine flu virus that spreads among humans in the United States and Mexico, where he makes 20 fatal cases identified.

The virus was "clearly a potential pandemic" and the evolution of the situation is "unpredictable," warned the Director General of WHO Margaret Chan at a press conference call at the organization's headquarters in Geneva .

The non-affected countries should "increase their vigilance," claimed Ms. Chan, a specialist pandemics, returned the same morning the United States.

For her, this "new virus" identified as H1N1 and transmitted to humans constitutes a "serious situation".

"The most worrying is that (the virus) is transmitted from man to man", told AFP spokesman for WHO, Thomas Abraham, saying he had transferred "in genes never encountered before. "

"This is the first time we see an avian strain, two strains pig and a human strain, has also raised the head of the U.S. Centers for Control and prevention of diseases, Dave Daigle, who works closely with WHO.

These characteristics, together with the fact that the disease affects an age unusual for a bird - "young healthy adults - are fears of a serious epidemic, recalling the fears caused by avian influenza.

Especially since pigs are considered by experts as "crucibles" ideals that may give rise to a highly virulent strain for humans by combining genetic material from swine flu and avian flu.

According to WHO, they have been "involved in the emergence of new influenza viruses responsible for two pandemics of the twentieth century.

In Mexico, where 40 deaths were suspicious and a little more than a thousand patients placed under observation, authorities are already talking about the term epidemic.

The alert was given in the two countries, Mexico and the United States, where emergency operations centers have been opened. As the WHO has activated its "operations center" for managing emergencies.

For WHO, which has decided to send a special mission to Mexico, time is running out because the disease spreads geographically.

In Mexico, three homes were identified: the main city in Mexico, with 20 deaths from the virus (18 according to WHO), the second in central San Luis Potosi (24 cases including three deaths) and a third smaller, on the border with the United States, Mexicali (four non-fatal cases).

Evidence of the spread, the strain detected in 12 fatal cases in Mexico is "genetically identical" to that found in California in the United States, neighboring country where eight non-fatal cases have been identified, said WHO.

Moreover, in addition to California and Texas, suspect cases were reported in New York, where 75 students with flu symptoms were subjected to analysts, according to CNN.

Given the urgency, the Mexican authorities have ordered the total closure of schools and universities in Mexico and its state. The mayor of the capital decided on a mass vaccination campaign. Vaccines, however, were not available Friday, according to testimony.

"Vaccination is the second step," said the spokesman of the WHO stating that "vaccine production (was) possible since the virus was found" but needed "some time ".

Meanwhile, Tamiflu, a drug based oseltamivir used against bird flu, is effective for this virus, he said.