Sep 2, 2009 2:54 PM by Associated Press
Since it first emerged in April, the global swine flu epidemic has sickened more than 1 million Americans and killed 500. It's also spread around the world, infecting tens of thousands and killing 2,000.
This summer, the virus has been surprisingly tenacious in the U.S., refusing to fade away as flu viruses usually do. Health officials are already seeing a surge of cases as schools reopen.
Swine flu is more of a threat to certain groups - children under 2, pregnant women, people with health problems like asthma, diabetes and heart disease. Teens and young adults are also more vulnerable to swine flu.
Ordinary, seasonal flu hits older people the hardest, but not swine flu. Scientists think older people may have some immunity from exposure years earlier to viruses similar to swine flu.