The World Health Organization says countries must not lower their guard in the response to the swine flu outbreak.

世界卫生组织建议各国不应该降低对猪流感爆发的警戒程度。

The WHO said it was too early for countries to lower their guard

Almost 900 cases had been confirmed across five continents, the WHO said, and authorities had to remain vigilant（警醒的，警惕的）.

Viruses increased and decreased in activity, it said, and it was too early to tell whether the outbreak had peaked where it emerged in Mexico.

The warning came after health officials in Mexico said that cases of the virus appeared to be declining.

In Mexico, just over 100 people are thought to have died from the swine flu strain, although only 19 cases have been confirmed.

But on Sunday Health Minister Jose Angel Cordova said that the virus appeared to have peaked between 23-28 April.

"The evolution of the epidemic（传染病，流行病） is now in its declining phase," he told a news conference.

The WHO said authorities should remain on alert.

The current "round of activity" might have peaked, WHO official Gregory Hartl said, but that did not mean it was over.

"There is a high possibility that this virus will come back, especially in colder periods," he said.

Health experts in the US, meanwhile, say swine flu could soon be present throughout their country, as cases have been confirmed in more than half of all states.

Outside Mexico, the effects of the virus do not appear to be severe（剧烈的，严重的）.

In other developments:

• El Salvador says it has confirmed its first two cases of the virus, AFP reports, citing the country's health minister

• Egypt says it will continue slaughtering¹ pigs as a precaution against（作为预防措施） swine flu, following clashes on Sunday with farmers that left 12 people injured

• A number of Mexicans remain under quarantine（被隔离） in cities in China, triggering a diplomatic row

'Widespread'

Late on Sunday - before the apparent confirmation² from El Salvador - the WHO said it had found 898 cases of the virus across 18 countries.

Person-to-person transmission has been confirmed in six countries.

In the US, the number of confirmed cases rose from 160 to 226. Officials said this was because the results of lab tests were now coming through, rather than because of a new surge in cases.

But an expert from America's Centers for Disease Control and Prevention said the virus was fairly widespread.

"Virtually all of the United States probably has this virus circulating now," Dr Anne Schuchat said. "That doesn't mean that everybody's infected, but within the communities, the virus has arrived."

She said she expected cases to become more severe and to lead to deaths. She stressed that this in itself would not be unusual as every year 36,000 people die in the US after contracting seasonal³ flu.

WHO food safety scientist Peter Ben Embarek, meanwhile, said increased surveillance was necessary after the virus was found to have infected pigs in Canada.

But he said there was no recommendation to（给的建议） cull⁴（拣选，剔除） animals, and pork remained safe to eat.

"From a consumer point of view there is no risk from consuming cooked pork products," he said.