While having a similar sounding name, parainfluenza infections are usually much milder than influenza illnesses. Parainfluenza illnesses commonly affect infants and young children but can also cause other infections. Parainfluenza type 1 often causes croup, and there are generally more cases in the fall of odd-numbered years. Type 2 can also cause croup and usually occurs more often in the fall but is less common than type 1 or type 3. Type 3 infections circulate primarily in the spring and early summer but can occur throughout the year, especially when types 1 and 2 are not circulating. Parainfluenza type 4 is not as seasonal as the other types. Symptoms of parainfluenza infections include fever, runny nose, and cough, and the virus can also cause croup or pneumonia. As with previous viruses, transmission is caused by close contact with someone infected and actively coughing or sneezing. Transmission can also occur by touching contaminated surfaces and your face.
Parainfluenza virus is an enveloped, negative-sense RNA virus in the Paramyxoviridae family. It is a larger virus ranging from 180–300 nm in diameter. This virus has H and N spikes similar to influenza, but there are only four known types (1–4) and two subtypes (4a and 4b).
