U.S. officials declared a public health emergency today over swine flu, now that 20 cases of the illness have been confirmed in the country, with 80 dead and 1,300 infected in Mexico.

Twenty cases—in California, Kansas, New York State and Texas, although none fatal—may not sound like a lot, but the U.S. Centers for Disease Control and Prevention (CDC) acting director Richard Besser told reporters in Washington, D.C., that is probably just the beginning. “We are seeing more cases of swine flu,” Besser said. “We expect to see more cases of swine flu. As we continue to look for cases, I expect we’re going to find them.”

So what is swine flu? Swine flu "is a respiratory disease of pigs caused by type A influenza viruses that causes regular outbreaks in pigs," according to the CDC. Humans are not usually affected, although such infections can happen. "Swine flu viruses have been reported to spread from person to person, but in the past, this transmission was limited and not sustained beyond three people."

The virus responsible for the current outbreak, however—strain H1N1—is contagious between humans, says the CDC, although it's unclear just how easily that happens. "Flu viruses are spread mainly from person to person through coughing or sneezing of people with influenza," the agency notes in a Q&A. "Sometimes people may become infected by touching something with flu viruses on it and then touching their mouth or nose."

Symptoms of the swine flu are the same as those of other types of flu: fever, cough, sore throat, body aches, headache, chills and fatigue, all of which may be more severe in those who are already sick or have chronic medical conditions. To prevent it, the CDC urges hand washing, plenty of sleep, and drinking plenty of fluids. (You can't get it from pork, if you're wondering, although you may recall that pigs have also now been found to carry "superbugs".)

There is no effective vaccine against swine flu at the moment, but the CDC recommends using Tamiflu (olsetamivir) or Relenza (zanamivir) to treat or prevent it. Tamiflu-maker Roche said today it was ready to deliver three million doses of Tamiflu, which is only available by prescription in the U.S., but typical flu viruses seem to be more and more resistant to the antiviral medication, as we've reported.

In 1976, with the lessons of the 1918 Spanish flu pandemic never far from their minds, U.S. health officials responded to the death of a private at Fort Dix from the swine flu by launching a campaign to vaccinate 220 milion Americans against swine flu. The 1976 pandemic never came, leading many, in hindsight, to question the decision to vaccinate, although the 1918 Spanish flu strain was similar and killed a half million people in the U.S. and more than 20 million around the world.

See our in-depth report for more on swine flu and other outbreaks and pandemics.