Although the H1N1 virus has not proven as deadly as the annual bouts of seasonal flu, the World Health Organization (WHO) raised the pandemic threat level today from 5 to 6, the highest designation, signaling that there has been substantial "community transmission" on multiple continents. 

At least 28,774 cases of the virus have been reported in 74 countries, and 144 people have died, the WHO reports on its Web site. For comparison, the seasonal flu kills an average of 36,000 people in the U.S. every year, according to the U.S. Centers for Disease Control (CDC).

"The world is now at the start of the 2009 influenza pandemic," WHO Director-General Margaret Chan said in a press conference today. "Further spread is considered inevitable." Now that the virus has been acknowledged as a global pandemic, it's more like "a marathon—you're not talking about a sprint," said another WHO spokesperson, who expects it to spread for the next couple years until people begin to develop immunity to the strain.

The good news, she said, is that the virus hasn't become any more virulent and appears to still be relatively stable, not showing signs of mutating yet, although any "influenza virus is full of surprises," she noted.

Most vaccine manufacturers are still churning out traditional seasonal flu vaccines, but they will soon switch over to a vaccine for H1N1, which won't be available until September, Chan said.

This flu continues to primarily infect people under the age of 25. However, unlike seasonal flu—which often hits elderly the hardest—as many as half of those with severe cases have been in people of middle age, said Chan. She also noted that most of those affected have had underlying health conditions.

The WHO does not recommend that countries impose travel restrictions, nor close borders. Chan did express concern about the virus's spread to developing countries, where monitoring and medical care might not be as thorough as many of the wealthier nations who have had the bulk of reported cases so far.

Nations across the globe have already been taking preventative measures. China has been detaining travelers who are sick or who may have been in contact with someone who was. One of the recent quarantined air passengers was New Orleans Mayor Ray Nagin, reported the Associated Press. "When you see people coming toward you with full hazmat gear on, it's pretty interesting," Nagin told the AP.

The WHO's pandemic alert level had been at 5, which entails sustained human-to-human transmission, since April 29. The other novel flu virus, avian flu H5N1, continues to circulate at a WHO global alert level of 3.

Visit our in-depth report on H1N1 (aka swine flu) to learn more about the virus and how to avoid getting it.