Primary influenza viral pneumonia develops soon (24 to 36 hours) after flu symptoms start and does not get better with antibiotics. It rarely causes death in young, healthy people, but it can often be life-threatening in older adults, people who have other diseases, and pregnant women.
Secondary bacterial pneumonia most often develops after a period of improvement following the flu. People with this type of pneumonia usually get better with antibiotics.
Bronchiolitis, which is an inflammation of the small air passages (bronchioles). Bronchiolitis affects infants and is the leading cause of serious lower respiratory illness.
Sinusitis, which is an infection or inflammation of the mucous membranes that line the inside of the nose and facial sinuses. Facial sinuses are hollow spaces, or cavities, located around the eyes, cheeks, and nose.
Croup, which is a swelling or obstruction in the windpipe (trachea). It causes a distinctive hoarseness and a barking cough, a high-pitched sound (stridor) heard when breathing in, and trouble breathing.
Reye syndrome, which is a very rare but serious disease that most often occurs in children 6 to 12 years old. The exact cause is not known. But it is associated with children who have recently had chickenpox (varicella) or flu (influenza) and have taken aspirin. The disease primarily targets the brain and liver and can cause drowsiness, confusion, seizures, coma, and in severe cases, death.
Inflammation of the heart muscle (myocarditis), inflammation of muscles (myositis), or inflammation of the sac around the heart (pericarditis).
Fatigue and a lack of energy that persist after flu symptoms are gone. People may take several weeks to fully recover, although no cause for the symptoms has been identified.