Here are answers to some frequently asked questions about swine flu.

The information is drawn from the data released by the World Health Organization in Geneva and the U.S. Centers for Disease Control and Prevention in Atlanta.

Q: What is swine flu?

A: Influenza is a virus that infects people, birds, pigs and other animals such as ferrets. Swine flu, or swine influenza, is a form of the virus that normally infects pigs. There are many forms of flu, and the different varieties have the ability to exchange genes with one another. The form of flu that originated in Mexico is a genetic mixture of viruses that have been seen in pigs, birds and people. It's being called a swine flu because the overall structure of the virus is of the type that affects pigs, said Keiji Fukuda, a WHO official.

Q: How do people catch swine flu?

A: Studies are ongoing about how this particular swine flu is transmitted. Flu is generally transmitted through the respiratory tract. Droplets of infected body fluids may carry flu when people cough or sneeze. Studies indicate that masks called N95 respirators, when properly used, filter germs from the breath and hamper the spread of flu. Neither contact with pigs nor eating pork has been linked to the spread of the flu, Fukuda said.

Q: What are the symptoms of swine flu?

A: About one to four days usually elapse between the time a person is infected and the onset of symptoms. Influenza normally causes symptoms such as coughing, sneezing, headaches and body aches, fever, chills, and sometimes vomiting and diarrhea. Swine flu causes the same symptoms, and may be difficult to distinguish from other strains of flu and respiratory illnesses. Severe cases of flu that lead to death are normally seen in very young and very old people whose immune systems are too weak to fight off the virus. Adults with severe illness may also have difficulty breathing, dizziness, confusion, or severe vomiting and diarrhea.

Q: Is there a vaccine against the swine flu that's now spreading?

A: Flu vaccines generally contain a dead or weakened form of a circulating virus. The vaccine prepares the body's immune system to fend off a true infection. For the vaccine to work, it must match the circulating, "wild-type" virus relatively closely. There is no vaccine currently that exactly matches the swine flu. The seasonal flu vaccine isn't effective against swine flu, said Richard Besser, acting head of the CDC.

Vaccine makers have contacted the World Health Organization about obtaining samples of the virus needed to make a vaccine. Making flu vaccine can take three to six months. No decision has been made to order a vaccine against swine flu, Besser said.

Q: How can I tell if my child is sick?

A: Children who are breathing abnormally fast or slowly may have respiratory illness. Bluish skin indicates a need for quick attention. Children who are abnormally sluggish and sleepy, irritable, or have fever or rash may also need attention.

Q: Have there been outbreaks of swine flu before?

A: Yes. Health officials said in 1976 that an outbreak of swine flu in people might lead to a pandemic. Widespread vaccination was carried out in the U.S. before experts determined that the virus was not dangerous enough to cause a pandemic. Swine flu occasionally infects people in the U.S. without causing large outbreaks. From 2005 through January 2009, there were 12 reported swine flu cases in the U.S. None of them caused deaths.

Q: Why are health officials concerned about the outbreak of swine flu?

A: When flu viruses mix genes with one another, they can take on new forms. New flu viruses are harder for the human immune system to defend against. With little or no opposition from the immune resistance, the virus can grow quickly and invade many tissues and organs. They may also set off a harmful immune overreaction in the body, called a "cytokine storm," that may be lethal in itself. The swine flu virus from Mexico may have the ability to spread quickly and kill people, possibly causing a worldwide pandemic, according to the WHO. Researchers are conducting studies to determine how easily the virus spreads in people and how dangerous it is.

Q: What's a flu pandemic?

A: A flu pandemic occurs when a new influenza virus spreads quickly and few people have immunity. While influenza viruses were only discovered about a century ago, researchers believe flu pandemics hit about two or three times each century. Some pandemics kill a few million people globally. The most severe flu pandemic on record was the 1918 Spanish Flu. Researchers estimate it killed about 50 million people around the world.

Q: Are there any similarities between the swine flu and earlier pandemic viruses?

A: Flu viruses are classified by two proteins on their surface, called H for hemagglutinin and N for neuraminidase. The swine flu found in Mexico and the 1918 Spanish Flu viruses are of the H1N1 subtype. Both viruses appear to have originated in animals. Researchers believe the Spanish Flu spread to people from birds. The two viruses are not identical, and there are still many genetic differences between them that researchers are studying.

Q: Do all H1N1 viruses cause pandemics?

A: No. H1N1 descendants of the Spanish Flu virus continue to circulate in people and sometimes cause outbreaks of seasonal flu.

Q: Are there drugs that treat swine flu?

A: Yes. Roche Holding AG's Tamiflu and GlaxoSmithKline Plc's Relenza both react against swine flu. The U.S. Department of Homeland Security has released 25 percent of its stockpile of Tamiflu and Relenza, according to Secretary Janet Napolitano. Flu viruses sometimes develop resistance to antiviral drugs. The human form of H1N1 seasonal flu that's currently circulating is resistant to Roche's Tamiflu (not Relenza). If the two viruses were to exchange genes, the swine flu might become resistant, too. The drugs should be administered within the first 48 hours of the onset of symptoms, according to the CDC.

Tamiflu and Relenza may also help prevent swine flu in people who have been exposed to someone who was sick.

Q: How else can I protect myself from swine flu?

A: Personal hygiene measures, such as avoiding people who are coughing or sneezing and frequent hand-washing, may prevent flu infection. Those who aren't health professionals should avoid contact with sick people. People who get sick with flu symptoms should stay home. Studies have suggested that closing schools, theaters, and canceling gatherings in the early stages of a pandemic can limit its spread. Such measures would likely take place if health officials determine that the virus is spreading quickly enough and is deadly enough to cause a pandemic.