The U.S. government has 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 a public health emergency," Secretary of Homeland Security Janet Napolitano said 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.

Countries around the world are watching for the virus, and scientists are scrambling to learn more about the virus and stop it before it becomes a pandemic.

Fukuda says the global health community is taking the swine flu threat "very seriously" but wants more information before deciding whether to raise the WHO's pandemic alert level from phase 3 to phase 4.

An influenza pandemic occurs when a new influenza type A virus emerges for which there is little or no immunity in the human population, begins to cause serious illness, and then spreads easily from person to person worldwide, according to background information from the U.S. Department of Health and Human Services.

Swine Flu Symptoms

Symptoms of swine flu seen in U.S. patients so far have been "relatively nonspecific -- high fever, cough, sore throat, muscle aches, possibly vomiting and diarrhea in some numbers," says Schuchat.

The problem is, those symptoms aren't unique to swine flu.

They "can be caused by so many different things," Schuchat says, which makes it "impossible" for a patient to tell if they have swine flu, as opposed to another flu virus or a different illness.

"This is a dilemma, a challenge, we're wrestling with," says Schuchat. She encourages patients to use their judgment about whether they're sick enough to see a doctor, and to definitely do so if they've recently been to a high-risk area, such as Mexico.

For more info and updates, visit: http://www.cdc.gov/swineflu/