Why can weakness linger after other influenza symptoms begin to improve?

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

It is a common misconception that influenza symptoms resolve entirely once the fever breaks; in reality, the recovery process involves distinct phases.

While the acute phase of influenza typically involves an abrupt onset of high fever, muscle aches, and fatigue, these systemic symptoms often begin to subside after three to four days. As the body moves past this initial stage, respiratory symptoms—such as coughing and nasal discharge—often become more prominent.1

The lingering weakness experienced during this period is frequently associated with the body’s ongoing recovery from the infection of the respiratory tract, where the virus has caused damage to the ciliated epithelial cells lining the trachea and bronchial tubes. This residual fatigue reflects the physiological toll of the immune response and the time required for the respiratory tissues to repair themselves.1