Human parainfluenza viruses (HPIVs) are the viruses that cause human parainfluenza. HPIVs are a paraphyletic group of four distinct single-stranded RNA viruses belonging to the Paramyxoviridae family. These viruses are closely associated with both human and veterinary disease. Virions are approximately 150–250 nm in size and contain negative sense RNA with a genome encompassing about 15,000 nucleotides. The viruses can be detected via cell culture, immunofluorescent microscopy, and PCR. HPIVs remain the second main cause of hospitalisation in children under 5 years of age for a respiratory illness (only Respiratory syncytial virus (RSV) causes more respiratory hospitalisations for this age group). Classification The first HPIV was discovered in the late 1950s. The taxonomic division is broadly based on antigenic and genetic characteristics, forming four major serotypes or clades, which today are considered distinct viruses. These include: HPIVs belong to two genera: Respirovirus (HPIV-1 & HPIV-3) and Rubulavirus (HPIV-2 & HPIV-4). Viral structure and organisation HPIVs are characterised by producing enveloped virions and containing single stranded negative sense RNA. Non-infectious virions have also been reported to contain RNA with positive polarity. HPIV genomes are about 15,000 nucleotides in length and encode six key structural proteins. The structural gene sequence of HPIVs is as follows: 3′-NP-P-M-F-HN-L-5′ (the protein prefixes and further details are outlined in the table below). 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. This has led to the "rule of six" being coined. Exceptions to this rule have been found and its exact advantages are not fully understood. Electrophoresis has shown that the molecular weight (MW) of the proteins for the four HPIVs are similar (with the exception of the phosphoprotein, which shows significant variation). Viral entry and replication Viral replication is initiated only after successful entry into a cell by attachment and fusion between the virus and the host cell lipid membrane. Viral RNA (vRNA) is initially associated with nucleoprotein (NP), phosphoprotein (P) and the large protein (L). The hemagglutinin–neuraminidase (HN) is involved with viral attachment and thus hemadsorption and hemagglutination. Furthermore, the fusion (F) protein is important in aiding the fusion of the host and viral cellular membranes, eventually forming syncytia. Initially the F protein is in an inactive form (F0) but can be cleaved by proteolysis to form its