FACTBOX: How swine flu spreads in humans

(Reuters) - A new strain of influenza is infecting people in Mexico and the United States and may have killed up to 60 people in Mexico, global health officials said on Friday.

The CDC has analyzed samples of the H1N1 virus from some of the U.S. patients, all of whom have recovered, and said it is a never-before-seen mixture of viruses from swine, birds and humans.

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 sicken pigs, not humans. Most cases occur when people come in contact with infected pigs or contaminated objects moving from people to 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 they can be passed from human to human. Transmission among humans is thought to occur in the same way as seasonal flu -- by touching something with flu viruses and then touching their mouth or nose, and through coughing or sneezing.

* 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 provide partial protection against swine H3N2, but not swine H1N1 viruses, 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 as it does other bacteria and viruses.

Editing by Maggie Fox