HPIVs are a paraphyletic group of four distinct single-stranded RNA viruses belonging to the Paramyxoviridae family.

[2] Virions are approximately 150–250 nm in size and contain negative sense RNA with a genome encompassing about 15,000 nucleotides.

The taxonomic division is broadly based on antigenic and genetic characteristics, forming four major serotypes or clades, which today are considered distinct viruses.

[3] HPIVs are characterised by producing enveloped virions and containing single stranded negative sense RNA.

[7] With the advent of reverse genetics, it has been found that the most efficient human parainfluenza viruses (in terms of replication and transcription) have a genome nucleotide total that is divisible by the number 6.

[8] Electrophoresis has shown that the molecular weight of the proteins for the four HPIVs are similar (with the exception of the phosphoprotein, which shows significant variation).

Viral RNA (vRNA) is initially associated with nucleoprotein (NP), phosphoprotein (P) and the large protein (L).

[12] The pathogenicity of HPIVs is mutually dependent on the viruses having the correct accessory proteins that are able to elicit anti-interferon properties.

[15] Upper respiratory infections (URI) are also important in the context of HPIV, however, they are caused to a lesser extent by the virus.

[17] Repeated infection throughout the life of the host is not uncommon and symptoms of later breakouts include upper respiratory tract illness, such as cold and a sore throat.

[citation needed] HPIV-3 has been closely associated with bronchiolitis and pneumonia, and principally targets those aged <1 year.

[3] Those with compromised immunity have a higher risk of infection and mortality and may fall ill with more extreme forms of LRI.

Infection can occur when infectious material contacts the mucous membranes of the eyes, mouth, or nose, and possibly through the inhalation of droplets generated by a sneeze or cough.

[12] Recent evidence suggests that the virus-specific antibody immunoglobulin E may be responsible for mediating the large-scale releases of histamine in the trachea that are believed to cause croup.

[12] Diagnosis can be made in several ways, encompassing a range of multi-faceted techniques:[4] Because of the similarity in terms of the antigenic profile between the viruses, hemagglutination assay (HA) or hemadsorption inhibition (HAdI) processes are often used.

Both complement fixation, neutralisation, and enzyme linked immunosorbent assays – ELISA, can also be used to aid in the process of distinguishing between viral serotypes.

[12] Numerous factors have been suggested and linked to a higher risk of acquiring the infection, inclusive of malnutrition, vitamin A deficiency, absence of breastfeeding during the early stages of life, environmental pollution and overcrowding.

[31] Maternal antibodies may offer some degree of protection against HPIVs during the early stages of life via the colostrum in breast milk.

[12] Ribavirin is a broad-spectrum antiviral, and as of 2012, was being administered to those who are severely immuno-compromised, despite the lack of conclusive evidence for its benefit.

[4] Environmental factors which are important for HPIV survival are pH, humidity, temperature and the medium within which the virus is found.

While both can cause upper respiratory symptoms, influenza is more likely to result in high fever, body aches, and fatigue.

Parainfluenza often produces milder, cold-like symptoms such as runny nose, cough, and low-grade fever.