The World Health Organization reported Wednesday that 22 countries have officially reported a total of more than 1500 cases of Influenza A H1N1 infection. Mexico continues to report the greatest number of cases (822), with more than 400 confirmed in the United States. The WHO has so far refrained from calling this a level six global pandemic, but one U.S. health official said he would not be surprised if that does not happen eventually.

You may have heard the newest influenza virus referred to as H1N1. Finally we hear what the "H" and "N" mean from Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

"We identify influenza by two proteins on its surface called Hemagglutinin and Neuraminidase and hence the terminology 'H' and 'N'," Dr. Fauci said.

Influenza viruses generally affect a population in seasons. Sometimes, according to Dr. Fauci, they can modify slightly in what's called a drift. Researchers often adjust the influenza vaccine for that drift.

But in the 20th century, there was a major shift in the influenza virus and scientists and the public were unprepared for it.

In 1918, the H1N1 virus, known then as the Spanish flu, killed 50 million people in a global pandemic.

Two other pandemics occurred in 1957 and 1968. The Asian Flu in 1957 killed at least one million and the Hong Kong Flu in 1968 killed 700-thousand.

With the spread of avian flu in 2004, health experts had practice in preparing for a possible pandemic on the scale of the current H1N1 flu.

They are not saying this virus is as deadly as the 1918 flu. But they warn that poorer countries need help to prepare.

Dr. Dennis Carroll, an influenza specialist with the U.S. Agency for International Development, says exercises have already begun in parts of Africa.  

"Country representatives from seven east African countries were able to draw from their experiences developing national pandemic plans to test them in real time context of a possible H1N1 pandemic," Dr. Carroll said. "Similar exercises already planned for south African countries in June and Asian countries in August.  

Scientists in the United States and elsewhere are now in developmental stages of a vaccine for this H1N1 virus.  

They say the virus could gather strength as the flu season begins in the southern hemisphere and as it sets in some months from now in in the northern hemisphere.

Health experts predict that the earliest date a vaccine would be available is in September.