Human parainfluenza virus: One in a group of four RNA viruses that rank second only to respiratory syncytial virus (RSV) as a common cause of lower respiratory tract disease in young children. Like RSV, human parainfluenza viruses (HPIVs) can cause repeated infections throughout life. These infections are usually manifested by an upper respiratory tract illness (such as a cold or sore throat). HPIVs can also cause serious lower respiratory tract disease with repeat infection (including pneumonia, bronchitis, and bronchiolitis), especially among the elderly, and among patients with compromised immune systems. There are four serotypes types of HPIV (1 through 4). No vaccine is currently available to protect against infection caused by any of the HPIVs.