Health officials' advice is to follow common-sense precautions: Wash your hands, stay home if you're sick and listen to your local health authorities.
"Very frequent hand-washing is something that we talk about time and time again and that is an effective way to reduce transmission of disease," Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said Sunday at a White House briefing.
"If you're sick, it's very important that people stay at home. If your children are sick, have a fever and flu-like illness, they shouldn't go to school. And if you're ill, you shouldn't get on an airplane or another public transport to travel. Those things are part of personal responsibility in trying to reduce the impact. "
"In areas with no disease yet, a lot of what we can do sounds simple and repetitive but helps," said Dr. Anne Schuchat, interim deputy director of the CDC's Science and Public Health Program. In addition to washing hands often, she recommends covering your mouth when coughing and sneezing and avoiding touching your eyes and nose in case the virus is on your hands.
By midday Sunday, there were 20 confirmed cases in the United States. All infected U.S. patients have recovered. No one has died.
Mexico, however, has been hard hit: 81 deaths had been deemed "likely linked" to a deadly new strain of the flu virus by health authorities there. Viral testing has confirmed 20 cases, said Dr. Jose A. Cordova Villalobos, Mexico's health secretary, and Mexican authorities are investigating at least 1,000 cases of illness.
Cases also have been reported in New Zealand and Canada.
The World Health Organization calls the situation a "public health emergency of international concern," and the United States on Sunday declared a "public health emergency," likened by Homeland Security Secretary Janet Napolitano to preparations for a potential hurricane. No authorities are calling the outbreak a pandemic.
The CDC expects U.S. numbers will grow and recommendations will change depending on what happens in individual communities, said the CDC's Schuchat. The public should pay attention to what is happening nearby and heed the guidance of local health officials regarding school, work and public events.
Because so much is still unknown at this point, the main risk factor is people traveling to areas where cases have already been identified. "However this virus may already be in other places in the United States," CDC spokesman Tom Skinner told CNN. "With enhanced surveillance, we will see more cases and that is why it makes good sense to be proactive, by doing things we know are effective in reducing exposure." Explainer: Swine flu facts »
Schuchat noted swine flu symptoms are relatively general and nonspecific. "So many different things can cause these symptoms. it is a dilemma," she said. "It is a challenge that we are wrestling with. There is not a perfect test right now to let a person or doctor know exactly what this is."
U.S. travel to Mexico has not been restricted, but Schuchat specifically advised anyone who feels ill after returning from Mexico to see a doctor.
Unfortunately, since this is a new strain of influenza, the flu vaccine for this past flu season offers no protection. "However, we do have anti-virals that work against this swine flu," said Skinner, referring to oseltamivir (Tamiflu) and zanamivir (Relenza). "Someone who has [swine flu], if they are treated early, the anti-flu medicines work against this."
According to Skinner, health officials are still trying to figure out where exactly the virus originated, how transmissible it is and why it is mild in some cases and deadlier in others.