Americas top infectious disease expert answers your questions:

The media still are reporting new cases of swine flu (H1N1), particularly those causing hospitalization or death. This gives the impression that swine flu is far more widespread and dangerous than it really is.

Fact: Hardly any of the dire predictions about swine flu have proved to be accurate. It has not infected large numbers of people. It has not resulted in high rates of hospitalization. It has not caused a larger-than-expected number of deaths.

To learn more about the swine flu, Bottom Line/Personal spoke with infectious disease expert William Schaffner, MD. He addressed some of your concerns in the June 1, 2009, issue. Here, he answers more of your questions…

How common is swine flu? As of June 11, 2009, the World Health Organization declared the global outbreak of swine flu to be a pandemic, a scary word that simply means that the virus has had continuous person-to-person spread on more than one continent. There have been 28,774 cases worldwide, including 144 deaths in 74 nations. The US has had 13,217 cases and 27 deaths.

To put this in perspective, up to 50 million Americans get “normal,” seasonal flu in an average year and about 36,000 die from it.

Isn’t swine flu more dangerous than regular flu? No. The rate of hospitalization in cases of confirmed swine flu is about 0.7 percent, roughly the same as that of seasonal flu. If anything, swine flu appears to be milder, if only because it mainly affects young adults, who are less likely to experience flu-related complications than older people.

Why are young people more at risk? About 64 percent of swine flu patients are between the ages of five and 24. Only about 1 percent of confirmed cases occurred in people 65 years old or older.

Possible reason: In general, children are the main flu “reservoirs.” They haven’t been exposed to as many viruses as older adults and thus are less likely to have protective antibodies.

In addition, millions of older adults have been exposed to related flu strains in the past. They developed antibodies that appear to offer at least partial protection against swine flu.

What about news reports that healthy young people’s systems “overreacted” to the swine flu virus, making them sicker? Occasionally a young person’s immune system will react strongly to a flu virus producing a “cytokine storm,” a potentially fatal immune reaction that inflames an infection.

Who is at risk for getting seriously ill? People with diabetes, heart disease, cancer and other diseases that weaken the immune system. Pregnant women also are at greater risk. These groups of people are at greater risk for getting seriously ill with seasonal flu, too.

New finding: Researchers noted that obese patients with swine flu were more likely to get seriously ill than thinner patients. This probably is because very large individuals have more difficulty expanding their lungs, which increases the risk for fluid buildup and pneumonia.

How contagious is swine flu? Epidemiologists (specialists who study diseases in populations) use the term “attack rate” to measure a virus’s transmissibility. Both swine flu and seasonal flu have attack rates of 25 to 40 percent. This means that in a given population, the illness will occur in 25 to 40 percent of individuals.

How is the disease transmitted? Flu mainly is spread by droplet transmission. Someone sneezes, coughs or merely exhales, and virus-filled droplets are expelled. If you inhale the virus or get it in your nose or eyes, there’s a good chance that youll develop the flu. The virus also can survive in eye and nose secretions on the hands.

Wash your hands often. It won’t eliminate the risk of getting swine flu, but it reduces the odds.

Can I get swine flu from doorknobs, countertops, etc.? Possibly. Flu viruses potentially can survive for a few hours on inanimate surfaces — but the risk is too low to even think about.

Will swine flu become more dangerous? The 1918–1919 flu pandemic, which killed up to 100 million people worldwide, was mild when it first appeared in spring–summer. The second wave (summer–fall) and third wave in the winter were far more lethal. Because this year’s swine flu started in the summer, some experts wonder if it might follow a similar pattern.

Flu viruses are unusually “plastic” — they readily recombine with other viruses, swap genetic material and develop new characteristics. H1N1 might mutate in the coming months and become more — or less — dangerous, or it might not change at all. There’s no way to predict this.

Good news: The H1N1 family of viruses tends to change less rapidly than other viruses. There’s a fair chance that the strains of swine flu that reemerge in the fall essentially will be the same virus that we have now.

Do face masks help? The surgeon’s masks that many people wore when the virus first emerged might have limited the spread somewhat. However, the Centers for Disease Control and Prevention doesn’t recommend them, because it is difficult to assess their potential effectiveness in community settings.

Exception: The N95 respirator, designed to be worn by workers in hazardous environments, can be tightly sealed around the face and probably is effective in protecting you from getting the infection. The problem is that these masks are hot and uncomfortable, and they make it difficult to breathe. They aren’t practical for daily life.

Should I see a doctor if I think I have swine flu? If you’re sick, call your doctor. Don’t risk infecting other people in the doctor’s office or emergency room. Many doctors will give flu-related advice — and sometimes a prescription for an antiviral drug — over the phone. In general, the only people who really need to worry are those who feel extremely ill and those with a high risk of getting the flu (such as those with impaired immunity or lung disease). If you fall into this category, have a prearranged plan with your doctor.

Can my doctor give me a test to determine if I have swine flu? No. The test that a private physician can give determines only whether the flu virus is type A, the most common type of flu virus, or type B. If you test positive for either type of flu at your doctor’s office, this means that you have a flu, but not necessarily the swine flu. At present, state labs doing swine flu tests are accepting specimens only from patients ill enough to be admitted to the hospital.

I’ve read that people died from the flu vaccine in 1976. How do I know that a new vaccine will be safer? It’s not known that the 1976 vaccine caused any deaths. It was given to more than 40 million Americans. During this time, a handful of deaths — along with a small number of cases of Guillain-Barré syndrome, an autoimmune disease that affects the nerves — was reported in patients who had received the vaccine. But in fact, the apparent complications were so rare that they might have arisen normally, without any link to the vaccine.

More than 100 million doses of flu vaccine are given every year. Fewer than 5 percent of patients will experience any side effects, and most of the side effects that do occur, such as pain at the injection site, are insignificant.

Swine Flu Vaccine

A vaccine against H1N1 is expected to be available sometime in the fall. If a swine flu vaccine is used this year, people probably will need three separate inoculations — one for seasonal flu, a “priming” dose for swine flu and then a swine-flu booster a month or so later.