The H1N1 flu virus (red) is pictured in this image taken July 9, 2009. The new H1N1 influenza virus bears a disturbing resemblance to the virus strain that caused the 1918 flu pandemic, with a greater ability to infect the lungs than common seasonal flu viruses, researchers reported Monday. REUTERS/Image courtesy of Yoshihiro Kawaoka/University of Wisconsin-Madiso/Handout

Here are some facts from the U.S. Centers for Disease Control and Prevention about how swine flu spreads in humans:

* Swine flu viruses typically cause illness in pigs, not humans. Most cases occur when people come into contact with infected pigs or contaminated objects moving between people and pigs.

* Pigs can catch human and avian or bird flu. When flu viruses from different species infect pigs, they can mix inside the pig and new, mixed viruses can emerge.

* Pigs can pass mutated viruses back to humans, and these can be passed from human to human. Transmission among humans is thought to occur in the same way as with seasonal flu -- by touching something contaminated with flu viruses and then touching one's mouth or nose, and through coughing or sneezing. One of the most effective prevention measures is regular hand washing.

* Symptoms of swine flu in people are similar to those of seasonal influenza -- sudden fever, coughing, muscle aches and extreme fatigue. This new strain also appears to cause more diarrhea and vomiting than normal flu.

* Vaccines are available to be given to pigs to prevent swine influenza. There is no vaccine to protect humans from swine flu, although the CDC is formulating one. The seasonal influenza vaccine may help to provide partial protection against swine H3N2, but not against swine H1N1viruses like the one circulating now.

* People cannot catch swine flu from eating pork or pork products. Cooking pork to an internal temperature of 160 degrees Fahrenheit (71 degrees Celsius) kills the swine flu virus along with other bacteria and viruses.