The World Health Organization has raised the swine flu pandemic alert level to phase 5 - just one step below the highest level. The decision comes as the number of countries with confirmed cases rises to at least 10.
World Health Organization Director-General Margaret Chan said the alert level was stepped up because influenza viruses are unpredictable and can spread quickly around the world.
"This change to a higher phase of alert is a signal to governments, to ministries of health and other ministries, to the pharmaceutical industry and the business community that certain actions should now be undertaken with increased urgency, and at an accelerated pace," said Margaret Chan.
The increase to level 5 indicates that there is sustained human-to-human transmission in communities in different geographical locations.
Dr. Chan said all countries should immediately activate their pandemic preparedness plans and countries should remain on high alert for unusual outbreaks of influenza-like illness and severe pneumonia.
"The biggest question, right now, is this: how severe will the pandemic be, especially now at the start?," she said.
The decision late Wednesday from the Geneva-based body comes the same day as Germany and Austria announced their first confirmed cases of swine flu, raising the total number of countries affected to 10. They include Mexico, the United States, Britain, Canada, Costa Rica, Israel, Spain and New Zealand. While Australia, France, Denmark and South Korea are investigating possible cases within their borders. Many of the infected persons had recently visited Mexico, which is the epicenter of the outbreak.
Assistant Director-General for Health Security and Environment at the World Health Organization, Dr. Keiji Fukuda, urged people to practice good hygiene, including washing their hands and covering their mouth when they cough, and if they become sick to stay home.
"If you are sick, if you have something that may well be swine flu, or any illness, it may be prudent to stay home until you are feeling better," said Keiji Fukuda. "This is a way of voluntarily preventing yourself from transmitting infection perhaps to others. It is also a way of getting rest and treatment. "
The most severe cases of swine flu have been found in Mexico, where the disease is suspected of killing more than 150 people and sickening hundreds more.
In the United States there have been more than 90 confirmed cases in 11 states. Earlier Wednesday, health officials announced the first swine flu fatality in the United States was a Mexican baby who was visiting relatives in the state of Texas.