Typical clinical features of influenza include:

• fever (usually 100 F-103 F in adults and often even higher in children)
• respiratory symptoms such as:
• cough
• sore throat
• runny or stuffy nose
• headache
• muscle aches
• and fatigue, sometimes extreme

Although nausea, vomiting, and diarrhea can sometimes accompany influenza infection, especially in children, gastrointestinal symptoms are rarely prominent. The term "stomach flu" is a misnomer that is sometimes used to describe gastrointestinal illnesses caused by other microorganisms. Novel H1N1 infections, however, have caused more nausea, vomiting, and diarrhea than the conventional (seasonal) flu viruses.

Most people who get the flu recover completely in one to two weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. In an average year, influenza is associated with about 36,000 deaths nationwide and many more hospitalizations. Flu-related complications can occur at any age; however, the elderly and people with chronic health problems are much more likely to develop serious complications after the conventional influenza infections than are younger, healthier people. However, the novel H1N1 virus had developed a different pattern of infection. Unfortunately, the pattern of infection is similar to that of the 1918 "Spanish flu" pandemic in which young people (pregnant individuals, infants, teens, and adults through age 49) are the most susceptible populations worldwide. Analysis of the people who were likely to develop complications from the H1N1 infection showed that other groups of people were also susceptible, including American Indians, patients with COPD, and obese individuals.

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