Influenza typically produces an abrupt onset of systemic and respiratory symptoms. Common symptoms include fever or feeling feverish, chills, cough, sore throat, runny or blocked nose, muscle or body aches, headache and pronounced fatigue. Unlike the common cold, influenza often makes a person feel substantially unwell over a relatively short period. However, symptoms vary considerably, and not everyone experiences every feature. In particular, absence of fever does not exclude influenza. Some infections are mild, and some people have no symptoms at all despite testing positive for influenza virus.
Fever and chills are common manifestations of influenza, although fever is not universal. Fever reflects the body's regulated increase in temperature in response to inflammatory signals generated during infection. Chills can accompany the process as the body adjusts to the higher temperature set point. Older adults and people with weakened immune systems may be less likely to develop the classic febrile response. Therefore, relying on fever alone to identify or exclude influenza can be misleading, especially in populations at increased risk of complications.
Muscle and body aches are particularly characteristic of influenza and can be substantial. They are associated with systemic inflammatory responses rather than simply mechanical effects of coughing or sleeping poorly. Headache and marked fatigue are also common. The combination of sudden onset, systemic aches, exhaustion, chills and respiratory symptoms often produces a clinical picture that is more debilitating than an uncomplicated cold. Nevertheless, symptom severity varies and other respiratory infections can also cause aches, so this pattern is suggestive rather than uniquely diagnostic.
Cough and sore throat are frequent respiratory manifestations. Influenza cough can be dry or non-productive and may be prominent or severe. Inflammation of the upper respiratory tract can produce throat discomfort, while infection and inflammatory effects elsewhere in the respiratory system can contribute to cough. A runny or stuffy nose can also occur, although nasal symptoms are generally less dominant than they are in many common colds. The coexistence of substantial systemic illness with respiratory symptoms is a useful clinical feature when considering influenza.
Some people with influenza, particularly children, experience gastrointestinal symptoms such as nausea, vomiting or diarrhoea. These symptoms are more common in children than adults and can occur alongside the respiratory manifestations rather than replacing them. Gastrointestinal symptoms can increase the risk of dehydration, especially when combined with fever and reduced fluid intake. Their presence does not automatically mean that the illness is gastrointestinal rather than respiratory because influenza can affect multiple physiological systems through viral infection and the host inflammatory response.
Influenza symptoms usually begin suddenly, contrasting with the more gradual onset typical of a common cold. Most uncomplicated influenza symptoms resolve within approximately three to seven days, although cough and fatigue can persist for more than two weeks, particularly in older adults and people with chronic lung disease. Many people recover within several days to less than two weeks, but recovery time varies. Persistent or worsening symptoms can indicate complications and should be considered in relation to the person's age, underlying conditions and overall clinical course.
Influenza can become complicated by pneumonia and other serious problems, so certain symptoms require urgent medical attention. In adults, warning signs include difficulty breathing or shortness of breath, persistent chest or abdominal pain or pressure, confusion, seizures, severe weakness, severe muscle pain, dehydration or symptoms that improve and then return or worsen. In children, concerning signs include breathing difficulty, bluish lips or face, chest retractions, dehydration, altered responsiveness and severe deterioration. These warning signs are not a complete list, and severe or rapidly worsening illness warrants prompt medical evaluation.
Symptoms alone cannot reliably distinguish influenza from COVID-19, the common cold and other respiratory infections. Testing can detect influenza viruses, and clinicians may also diagnose influenza based on symptoms and the local epidemiological context. This distinction matters because antiviral treatment can be appropriate for influenza, particularly in people at increased risk of severe disease, and works best when started promptly. Influenza vaccination remains an important preventive measure, while infection-control practices such as staying home when acutely ill, improving ventilation and practising respiratory and hand hygiene reduce opportunities for transmission.
