Respiratory infection caused the most deaths in 1999.
The proportion of respiratory samples that tested positive for influenza peaked during the 1999–2000 flu season at 33%, while the state and territorial epidemiologists' reports peaked at 44 states reporting either regional or widespread influenza activity.
It was originally discovered in military soldiers in the US in the spring of 1918.
One third of the world's population, or roughly 500 million people, are thought to have contracted this virus.
The lungs, nose, and throat are affected by the flu. High-risk groups include young children, elderly individuals, pregnant women, persons with chronic illnesses, and those with weakened immune systems.
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