Measles virus (MeV) is a single-stranded, negative-sense, enveloped (non-segmented) RNA virus of the genus Morbillivirus within the family Paramyxoviridae. Humans are the natural hosts of the virus; no animal reservoirs are known to exist. Hemagglutinin glycoprotein attaches the virus to cell receptors and thereby initiating infection. Binding of H protein to the receptor induces a conformational change that allows the F protein to trigger virion/cell membranes fusion. May use human CD46 and/or SLAMF1 as receptors for viral entry into the cell. The high degree of interaction between H and MCP/CD46 results in down-regulation of the latter from the surface of infected cells, rendering them more sensitive to c3b-mediated complement lysis.