April 26, 2009 -- The U.S. government today declared the swine flu outbreak a public health emergency. Swine flu has sickened at least 20 people in the U.S., by the CDC's latest count.

"We are declaring today a public health emergency," Secretary of Homeland Security Janet Napolitano said today at a White House news briefing. That declaration is "standard operating procedure," Napolitano said. "It is similar to what we do when we see a hurricane approaching a site. The hurricane might not actually hit but allows you to take a number of preparatory steps. We really don't know ultimately what the size or seriousness of this outbreak is going to be."

As part of the emergency, the Department of Homeland Security is releasing 25% of stockpiled antivirals -- Tamiflu and Relenza-- to the states.

Here's what officials want you to do: Stay home if you're sick, avoid close contact with people who are sick, wash your hands often, avoid touching your eyes, nose, and mouth, cover your mouth or nose with a tissue when coughing or sneezing, and keep up with health information in your own community.

The CDC has received reports of lab-confirmed swine flu cases in eight people in New York City, seven people in California, two in Texas, two in Kansas, and one in Ohio.

All of those swine flu cases have been relatively mild, although one person was briefly hospitalized, according to Keiji Fukuda, MD, assistant director-general for health security and environment at the World Health Organization.

The eight swine flu cases in New York City involved students at Saint Francis Preparatory School in Queens. All have recovered fully, according to a news release from the New York City Department of Health and Mental Hygiene.

So far, U.S. cases of swine flu have been milder than those seen in Mexico, where the World Health Organization has confirmed that at least 20 people have died from swine flu; health officials are investigating dozens more deaths in Mexico.

More swine flu cases are likely in the U.S. as public health officials heighten their hunt for the new strain of swine flu virus, notes Anne Schuchat, MD, the CDC's interim deputy director for science and public health program. Her advice: Be prepared for the possibility that there may be severe cases, and even fatalities, in the U.S.