This is the VOA Special English Health Report.

More than twenty countries have found cases of the new H1N1 influenza virus. In all around one thousand five hundred cases and thirty deaths have been confirmed. But most have been reported in Mexico and, to a much lesser extent, the United States. The virus was discovered in Mexico in the middle of March.

On Tuesday, federal officials in the United States advised schools not to close anymore when they have cases. The new health secretary, Kathleen Sebelius, said the virus does not seem to be as severe as once thought.

Women wearing masks on a bus in Mexico City on Tuesday

The World Health Organization will not declare an influenza pandemic unless a virus becomes widespread in at least two areas of the world. The W.H.O. is advising people to wash their hands often with soap and water. And it says there is no risk of infection from eating well-cooked meat from pigs.

The virus is commonly called swine flu because it contains some genetic material from pigs. But it also contains material from humans and birds.

The World Health Organization, the United Nations health agency, uses a system of six phases to warn of possible influenza pandemics. The current system has been used since two thousand five. For the first time the W.H.O. raised its alert level from phase four to phase five on April twenty-ninth.

Phase four means that a virus has gained the ability to cause community-level outbreaks. Phase five is marked by human-to-human spread of the virus into at least two countries in the same area of the world. This declaration is meant to warn that the time to plan for a pandemic is short.

Phase six would mean a global pandemic. Still, this would only mean that there are community-level outbreaks in at least one other country in a different area of the world.

The threat of H1N1 may have eased in recent days. But public health officials point out that flu viruses can return more aggressively after a few months. That happened with the so-called Spanish flu of nineteen eighteen which killed millions worldwide.

In the fourteenth century, ships that arrived in Venice, Italy, from areas infected with bubonic plague had to stay outside port for forty days. That was called quarantine, from a Latin word that means forty. Quarantine measures separate people from the general population if there are worries that they could be infectious.

And that's the VOA Special English Health Report, written by Caty Weaver. For the latest news, go to voaspecialenglish.com. I'm Mario Ritter.