Some recommendations that were legitimate a few months ago have been superseded by larger studies, giving some people reason to doubt the latest advice.

There is general agreement among experts about treating and preventing the disease, however. The following is based on the most up-to-date information.

Question: What's the difference between flus?

Answer: "Seasonal flu" actually consists of several strains. The Type A strains change slightly every year, which is why flu vaccine is reformulated annually. The population retains some immunity, however, so most people don't get seriously ill.

Every once in a while, a major genetic shift introduces a Type A strain to which few people have any immunity. This happened three times in the last century, causing the pandemics that began in 1918, 1957, and 1968. As the population developed immunity over several years, the pandemic strains settled into a seasonal flu pattern.

A new pandemic strain appeared again in spring 2009 and is now building toward its first full season.

Q: Is swine flu more dangerous than seasonal flu?

A: The new flu appears to carry about the same overall risk of complications and death as seasonal influenza. But far fewer people have immunity. An estimated 36,000 Americans a year die from complications of seasonal flu, so if twice as many get sick, there could be 72,000 deaths.

Q: Why are more young people dying from swine flu?

A: Older people have more immunity, apparently because the new flu is distantly related to strains that circulated through much of the last century.

Younger people with less immunity get the most exposure to this flu, spreading it quickly in school. "They are close to each other, they wipe their nose on their hands, they touch their friends," said Neil Fishman, head of infectious disease at the Hospital of the University of Pennsylvania.

Eighty-six children under 18 have died of swine flu, half of them since Aug. 30, the U.S. Centers for Disease Control and Prevention reported on Friday.

Q: Are pregnant women dying in large numbers?

A: Pregnant women are at high risk of complications. Pregnant women with swine flu were admitted to intensive-care units in Australia and New Zealand at nine times the expected rate given their proportion of the population, researchers reported this month in the New England Journal of Medicine.

Many scientists believe that pregnancy naturally lowers the body's infection-fighting mechanism in order to protect the fetus. And in late stages of pregnancy, the developing fetus pushes up against the lungs; diminished lung capacity raises the risk of respiratory complications if she gets the flu.

Q: Who else is at high risk?

A: People who are very obese — a body mass index greater than 35 (5-foot-9 and 240 pounds, for example) — also experience pressure on their lungs, plus they often have other medical conditions. The New England Journal study found that they were five times as likely to be admitted to the ICU for swine flu.

People with asthma or other chronic pulmonary disease have diminished lung function and, if they are on steroid medication, decreased immunity. The ICU report found their admissions to be twice the norm; a CDC study found that they made up 34 percent of all adult hospitalizations for swine flu.

People with diabetes, heart conditions, sickle cell anemia and any immunosuppressive disorder were also at greater risk.

Q: So are healthy adults at less risk?

A: Less, yes. But preliminary CDC data found that more than 40 percent of hospitalized adults had no known underlying condition (severe obesity was not counted).

Q: I feel like the vaccine is being pushed down my throat. Aren't there other ways to prevent infection?

A: Frequent and thorough handwashing (or use of alcohol-based sanitizer) and sneezing into your sleeve help a lot. So does staying home when you're sick and keeping a distance from others who seem sick. Eating well and getting enough sleep helps the immune system.

But behavior is difficult to change. Vaccines — like seat belts — afford protection regardless of how you or others act. They have saved millions of lives.

Q: How is the flu shot different from the nasal spray?

A: The injectable vaccine contains pieces of killed virus that provoke the immune system to respond.

The FluMist is made from a weakened form of live virus. It can reproduce in the cool nasal passages, stimulating an immune response, but cannot survive to cause illness in the warmer respiratory tract.

Q: How do I choose?

A: Shipments of FluMist began first, so it may be more available in the beginning. It also may be slightly more effective (and perhaps more pleasant). But the nasal spray is approved for use only in healthy, nonpregnant people ages 2 to 49.

The shot can be given to anyone over 6 months old.

Q: Can the vaccine cause the flu?

A: It is virtually impossible to get the flu from the vaccine. But the nasal spray can cause side-effects such as runny nose, headache, sore throat and low fever. And it is possible to contract the flu just before or just after getting the vaccine, and blame it for your misfortune.

Q: Does the vaccine prevent all cases of swine flu?

A: No. Both types protect against moderate to severe disease by limiting the degree of virus replication. The nasal spray may do better at preventing mild infection.

Q: How quickly does the vaccine become effective?

A: An immune response is detectable in three to five days and peaks at seven to 10 days.

Q: The vaccine is so new, how can they know it's safe?

A: The only thing that's new is the strain of virus that was used to make it. That happens every year with the seasonal flu vaccine.

Q: Were shortcuts made in manufacturing and testing?

A: "We have cut no corners," said CDC director Thomas Frieden. "This flu vaccine is made as flu vaccine is made each year. By the same companies. In the same production facilities. With the same procedures. With the same safety, safeguards. We have had literally hundreds of millions of people vaccinated against flu with flu vaccine made in this way."

Q: Does swine flu vaccine contain thimerosal or adjuvants?

A: The nasal spray is not made with thimerosal. Injectable vaccine comes in two types of packages. Multi-dose vials use thimerosal as a preservative that prevents contamination. Parents can ask for single-dose vials, which contain trace amounts.

No flu vaccine currently sold in the United States contains adjuvants, which can be used to boost effectiveness. The CDC did purchase some for possible emergency use in the future.

Q: I had swine flu over the summer. Do I need vaccine?

A: The term flu is used loosely, and often refers to a cold. Flu — swine or otherwise — is different. "You can often remember the hour that your illness hit, like it hit you on the head," said vaccine expert Paul A. Offit, chief of infectious diseases at Children's Hospital of Philadelphia.

If your summer affliction was a laboratory-confirmed Type A influenza, "you can assume that it was swine," he said, and "you wouldn't need the swine flu vaccine."

Q: What about the seasonal flu vaccine?

A: Most doctors say the seasonal flu vaccine is even more important because so many people may be out with swine flu.

Q: Can I get both at once?

A: You can receive swine and seasonal flu shots at the same time or one shot and one nasal spray of either type. But two nasal sprays must be separated by nearly a month.

Q: If I get sick, how do I know if I need medical help?

A: If you have an underlying medical condition and think you contracted the flu, it is important that you see a doctor. If you are otherwise healthy, call the doctor for advice.

If you are having difficulty breathing — or if your sick child is difficult to wake, is turning blue or grayish, or seemed to be getting better and then gets worse — seek medical help immediately.