Influenza Learning Path

Signs and symptoms of influenza

Influenza (flu) typically causes a sudden onset of symptoms that are often more severe than a common cold. The incubation period is usually 1–4 days (most commonly 1–2 days). Common early symptoms include fever, chills, headache, muscle pain, malaise, loss of appetite, low energy, and sometimes confusion. Respiratory symptoms commonly accompany these, especially a dry cough (the most common symptom), sore or dry throat, hoarse voice, and a stuffy or runny nose. Gastrointestinal symptoms such as nausea, vomiting, and diarrhea can occur, particularly in children. Symptoms usually last about 2–8 days, and many infections are mild and limited to the upper respiratory tract. However, influenza can progress to pneumonia, either from the initial viral infection (primary viral pneumonia) or from a secondary bacterial infection (secondary bacterial pneumonia). Primary pneumonia often shows rapid worsening of fever and cough with labored breathing and low oxygen levels, while secondary pneumonia may involve a period of improvement followed by recurrent fever, sputum production, and fluid buildup in the lungs. Influenza can also lead to serious complications such as acute respiratory distress syndrome (ARDS), meningitis, encephalitis, and worsening of existing conditions like asthma and cardiovascular disease.

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Incubation period and symptom duration

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