DefinitionParainfluenza refers to a group of viruses that lead to upper and lower respiratory infections.Causes, incidence, and risk factorsThere are four types of parainfluenza virus, all of which can cause upper respiratory infections or lower respiratory infections (pneumonia) in adults and children. The virus is responsible for approximately 40-50% of croupcases and 10-15% of bronchiolitis and bronchitis cases and some pneumonias.The exact number of cases of parainfluenza is unknown but suspected to be very high. Sometimes the viruses cause only a runny nose and other symptoms that may be diagnosed as a simple cold rather than parainfluenza.Infections are most common in fall and winter. Parainfluenza infections are most severe in infants and become less severe with age. By school age,