The flu is caused by influenza viruses, of which there are two types (A and B). Both influenza A and B viruses circulate throughout populations of humans, and are the typical cause of ordinary, seasonal spikes in flu rates. Influenza A viruses are also found in other animals — if they transfer to humans, they can cause flu pandemics, such as bird or swine flu.[1]
What causes the flu?
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•Last Updated: May 16, 2025
The flu is caused by influenza viruses, specifically types A and B, which circulate among humans and lead to seasonal increases in flu cases. Influenza A viruses can also infect other animals and may cause pandemics if they transfer to humans.