Influenza viruses are spread from person to person primarily through the coughing and sneezing of infected persons. The typical incubation period for influenza is 1–4 days, with an average of 2 days. Adults can be infectious from the day before symptoms begin through approximately 5 days after illness onset. Children can be infectious for longer than 10 days, and young children can shed virus for several days before their illness onset. Severely immunocompromised persons (e.g., patients with HIV infection) can shed virus for weeks or months.
Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms, including the following:
• Dry, nonproductive cough
• Extreme tiredness
• Fever (usually high)
• Muscle aches (myalgia)
• Runny or stuffy nose (rhinitis)
• Sore throat
Stomach symptoms, such as nausea, vomiting, and diarrhea, also can occur, but are more common in children than adults. Among children, otitis media, nausea, and vomiting are also commonly reported with influenza illness.
Influenza illness typically resolves after 3–7 days for the majority of persons, although cough and malaise can persist for longer than 2 weeks. Among certain persons, influenza can exacerbate underlying medical conditions (e.g., pulmonary or cardiac disease), lead to secondary bacterial pneumonia or primary influenza viral pneumonia, or occur as part of a co-infection with other viral or bacterial pathogens.
Young children with influenza infection can have initial symptoms mimicking bacterial sepsis with high fevers, and less than 20% of children hospitalized with influenza can have febrile seizures. Influenza infection has also been associated with serious conditions, such as the following:
• Encephalopathy (brain degeneration)
• Transverse myelitis (spinal cord inflammation)
• Reye syndrome (rare condition that may follow an upper respiratory infection)
• Myositis (muscle inflammation)
• Myocarditis (inflammation of the heart walls)
• Pericarditis (inflammation of the sac that surrounds the heart)