Measles morbillivirus (MeV), also called measles virus (MV), is a single-stranded, negative-sense, enveloped, non-segmented RNA virus of the genus Morbillivirus within the family Paramyxoviridae. It is the cause of measles. Humans are the natural hosts of the virus; no animal reservoirs are known to exist. Disease The virus causes measles, a highly contagious disease transmitted by respiratory aerosols that triggers a temporary but severe immunosuppression. Symptoms include fever, cough, runny nose, inflamed eyes and a generalized, maculopapular, erythematous rash and a pathognomic Koplik spot seen on buccal mucosa opposite to lower 1 st and 2 nd molars . The virus is spread by coughing and sneezing via close personal contact or direct contact with secretions. Replication cycle Entry The measles virus has two envelope glycoproteins on the viral surface – hemagglutinin (H) and membrane fusion protein (F). These proteins are responsible for host cell binding and invasion. The H protein mediates receptor attachment and the F protein causes fusion of viral envelope and cellular membrane. Additionally, the F protein can cause infected cells to directly fuse with neighboring uninfected cells forming syncytia. Three receptors for the H protein have been identified to date: complement regulatory molecule CD46, the signaling lymphocyte activation molecule (SLAMF1) and the cell adhesion molecule Nectin-4. For wild type and vaccine strains, extracellular domains of CD150 (SLAM or SLAMF1) and/or of nectin-4 (also called Poliovirus-Receptor-Like 4 (PVRL4)) mainly work as cell entry receptors. Minor fraction of wild type virus strains and all modern vaccine strains derived from the Edmonston strain also use CD46. Genome replication and viral assembly Once the virus has entered a host cell, its strand of negative sense ssRNA (single stranded RNA) is used as a template to create a positive sense copy, using the RNA-dependent RNA polymerase that's included in the virion. Then this copy is used to create a new negative copy, and so on, to create many copies of the ssRNA. The positive sense ssRNA is then mass translated by host ribosomes, producing all viral proteins. The viruses are then assembled from their proteins and negative sense ssRNA, and the cell will lyse, discharging the new viral particles and restarting the cycle. Genome and virion structure The RNA genome of the virus codes 6 main proteins Nucleoprotein (N), Phosphoprotein (P), Matrix protein (M), Fusion protein (F), Hemagglutinin (H), and Large Protein (L), which represents RNA dependent RNA polymerase (RdRp). The viral