A member of the genus Paramyxovirus of the family Paramyxoviridae which is associated with a variety of respiratory illnesses. The virus particles range in size from 90 to 200 nanometers, agglutinate red blood cells, and (like the influenza viruses) contain a receptor-destroying enzyme. They differ from the influenza viruses in their large size, their possession of the larger ribonucleoprotein helix characteristic of the paramyxoviruses, their tendency to lyse as well as agglutinate erythrocytes, and their generally poor growth in eggs.

Four subgroups are known, designated parainfluenza 1, 2, 3, and 4. Types 1, 2, and 3 are distributed throughout the world, but thus far type 4 has been found only in the United States. Parainfluenza 1 and 3 are ubiquitous endemic agents producing infections all through the year. Types 2 and 4 occur more sporadically. With all of the parainfluenza viruses, most primary infections take place early in life. About half of the first infections with parainfluenza 1, about two-thirds of those with parainfluenza 2, and three-fourths of those with parainfluenza 3 produce febrile illnesses. The target organ of type 3 is the lower respiratory tract, with first infections frequently resulting in bronchial pneumonia, bronchiolitis, or bronchitis. Type 1 is the chief cause of croup, but the other types have also been incriminated, to the extent that one-half of all cases of croup can be shown to be caused by parainfluenza viruses. Animal virus Influenza