How long do fever and major influenza symptoms usually last?

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

The onset of influenza symptoms is typically abrupt, following an incubation period of one to two days. During the initial phase, individuals experience a rapid rise in temperature to 38–40 °C (101–104 °F), accompanied by severe muscle aches, fatigue, and headaches.1

These major symptoms, including the fever, generally last for three to four days, after which the temperature begins to fall and recovery commences. Following this period, respiratory symptoms such as coughing and nasal discharge often become more prominent and may be accompanied by lingering feelings of weakness.1