Complications

The most serious complication of influenza is pneumonia-an infection of the lungs. Pneumonia can be caused by the influenza virus itself, or by bacteria that are able to enter the lungs because natural defences have been weakened by the flu. Anyone with persistent symptoms beyond three or four days merits medical attention. In addition, marked difficulty breathing or chest pain associated with coughing at any point during the illness should prompt a visit to the doctor.

Reye syndrome is a severe illness that can follow use of aspirin-containing products for viral infections. It tends to affect children, and has been associated with influenza A and chicken pox. Reye syndrome affects the liver and the brain, and is fatal in roughly 10% of cases. A child's risk for developing this disease is markedly lowered by avoiding aspirin use whenever they have a viral infection.