In the United States, human parainfluenza viruses (HPIVs) commonly cause respiratory tract illnesses.
There are four types of HPIVs and two subtypes that circulate at different times of the year.
- HPIV-1 infections often cause croup in children. There are usually more cases in the fall of every other year.
- HPIV-2 infections can also cause croup. HPIV-2 infections occur more commonly in the fall of every other year during the years when HPIV-1 is low. It is less frequently detected than HPIV-1 and HPIV-3.
- HPIV-3 infections usually occur in spring and early summer months each year. However, HPIV-3 infections can occur throughout the year, particularly when HPIV-1 and HPIV-2 are not in season.
- HPIV-4 (subtypes 4a and 4b) seasonal patterns are not as well characterized, but appear to occur in fall and winter each year.