Officials reported 20 U.S. cases of swine flu in five states so far, with the latest in Ohio and New York. Unlike in Mexico where the same strain appears to be killing dozens of people, cases in the United State have been mild — and U.S. health authorities can't yet explain why.

"As we continue to look for cases, we are going to see a broader spectrum of disease," predicted Dr. Richard Besser, acting chief of the Centers for Disease Control and Prevention. "We're going to see more severe disease in this country."

At a White House news conference, Besser and Homeland Security Secretary Janet Napolitano sought to assure Americans that health officials are taking all appropriate steps to minimize the impact of the outbreak.

Top among those is declaring the public health emergency. As part of that, Napolitano said roughly 12 million doses of the drug Tamiflu will be moved from a federal stockpile to places where states can quickly get their share if they decide they need it. Priority will be given to the five states with known cases so far: California, Texas, New York, Ohio and Kansas.

Napolitano called the emergency declaration standard operating procedure — one was declared recently for the inauguration and for flooding. She urged people to think of it as a "declaration of emergency preparedness."


Past flu pandemics

1918:
The Spanish flu pandemic that started in 1918 was possibly the deadliest outbreak of all time. It was first identified in the U.S., but became known as the Spanish flu because it received more media attention in Spain than in other countries, which were censoring the press during World War I. The 1918 flu was an H1N1 strain — different from the one currently affecting Mexico and the U.S. — and struck mostly healthy young adults. Experts estimate it killed about 40 to 50 million people worldwide.

1957:
The 1957 pandemic was known as the Asian flu. It was sparked by an H2N2 strain and was first identified in China. There were two waves of illness during this pandemic; the first wave mostly hit children while the second mostly affected the elderly. It caused about 2 million deaths globally.

1968:
The most recent pandemic, known as the Hong Kong flu, was the mildest of the three pandemics this century. It was first spotted in Hong Kong in 1968 and it spread globally over the next two years. The people most susceptible to the virus were the elderly. About 1 million people are estimated to have been killed by this pandemic, an H3N2 flu strain.