Every year, usually between October and May, 5 to 20 percent of the US population becomes ill with the flu. Seasonal flu is a respiratory illness that is easily transmitted from person to person. Most cases are not severe, but the flu can cause serious illness — and even death — in the very young, the elderly, and those with chronic illnesses. Most people have some immunity, but health care providers recommend annual flu shots, especially for those at risk.
Avian flu is a virulent viral disease that occurs naturally among wild birds. The H5N1 variant is deadly to domestic fowl, and it can be transmitted from birds to humans. It has caused a small number of cases of flu and even some deaths in people who have been in direct contact with infected birds. No cases of sustained human–to–human transmission have been established as of October 2006; however, scientists are concerned that the avian flu virus may mutate and become transmissible between humans. As of now, the avian flu virus has not been found in the United States. There is no human immunity and no vaccine is available.
In 1918, 1957, and 1968 the flu season in the US was especially severe, resulting in a much higher number of illnesses and deaths. Pandemic flu is a virulent human flu that triggers a global outbreak, or pandemic, of serious illness. Because there is little natural immunity, the disease can spread easily from person to person. Currently, there is no pandemic flu, but public health experts believe that a flu pandemic is likely to occur again in the future. Scientists worry that a mutant form of avian flu, under certain circumstances, could eventually cause a flu pandemic.