Human respiratory syncytial virus are found within the genus Orthopneumovirus, family Pneumoviridae, order Mononegavirales. Structurally, human respiratory syncytial virus is an enveloped, spherical virus with a diameter of approximately 150 nm. The RNA genome is packaged into the viral particle as a nonsegmented negative-sense molecule and codes for key internal structural proteins (matrix protein [M] and nucleoprotein [N]), proteins required for a functional polymerase complex (phosphoprotein [P] and polymerase [L]), nonstructural proteins involved in evasion of the innate immune response (NS-1 and NS-2), externally exposed transmembrane glycoproteins (small hydrophobic protein [SH], glycoprotein [G], and fusion protein [F]), and the regulatory M2 proteins (M2-1 antitermination protein and M2-2, involved in transcription/replication regulation).