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Why can influenza cause chills, fever, fatigue, and muscle aches?

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Influenza virus — influenza A colored transmission electron micrograph showing influenza viruses (red) at the outer surface of a host cell.

Influenza symptoms such as fever, chills, fatigue, and muscle aches are primarily the result of the body’s immune system responding to the virus as it infects and destroys ciliated epithelial cells in the respiratory tract.1

When the body encounters a new influenza strain, the immune system may trigger a robust inflammatory response. This process involves the release of signaling proteins known as cytokines. While these substances are essential for fighting infection, an overproduction of cytokines can lead to systemic inflammation, which manifests as the characteristic "flu-like" symptoms of high fever, severe muscle aches, and profound fatigue.21

The onset of these symptoms is typically abrupt, occurring one to two days after exposure, as the body rapidly elevates its temperature to between 38 and 40 °C (101–104 °F) to combat the viral invasion.13