But the H1N1 swine flu is not the usual "drift variant" of H1N1. It's come to humans from a different line of evolution. That means very few people have any natural immunity to H1N1 swine flu. The normal seasonal flu shot offers most people no protection against this new virus.

Some people who may have had seasonal H1N1 flu before 1957 might have a little bit of protective immunity against the new virus. That's because seasonal H1N1 flu strains that circulated before 1957 (and which were replaced by the 1957 pandemic flu bug) were genetically closer to the 2009 H1N1 swine flu. This protection, if it truly exists, is not complete. While relatively few elderly people have had H1N1 swine flu, many of those who did get the disease became severely ill.

What are swine flu symptoms?

Symptoms of H1N1 swine flu are like regular flu symptoms and include fever, cough, sore throat, runny nose, body aches, headache, chills, and fatigue. Many people with swine flu have had diarrhea and vomiting. Nearly everyone with flu has at least two of these symptoms. But these symptoms can also be caused by many other conditions. That means that you and your doctor can't know, just based on your symptoms, if you've got swine flu. Health care professionals may offer a rapid flu test, although a negative result doesn't mean you don't have the flu. The accuracy of the test depends on the quality of the manufacturer’s test, the sample collection method, and how much viral sample a person is shedding at the time of testing.

Like seasonal flu, pandemic swine flu can cause neurologic symptoms in children. These events are rare, but, as cases associated with seasonal flu have shown, they can be very severe and often fatal. Symptoms include seizures or changes in mental status (confusion or sudden cognitive or behavioral changes). It's not clear why these symptoms occur, although they may be caused by Reye's syndrome. Reye's syndrome usually occurs in children with a viral illness who have taken aspirin -- something that should always be avoided.

Only lab tests can definitively show whether you've got swine flu. State health departments can do these tests. But given the large volume of samples coming in to state labs, these tests are being reserved for patients with severe flu symptoms. Currently, doctors are reserving antiviral drugs for people with or at risk of severe influenza.

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