Transmission Electron Micrograph (TEM) of what happens to a group of living cells when invaded by the measles virus. The cells have clumped together into one giant, or syncytial cell, with all the cytoplasm fused into a single mass and the nuclei clustered together. These cells will eventually die under the virus attack. 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. The measles virus is the cause of measles, an infection of the respiratory system. Symptoms include fever, cough, runny nose, red eyes and a generalized, maculopapular, erythematous rash. The virus is highly contagious and is spread by coughing and sneezing via close personal contact or direct contact with secretions. Rubella (German measles) and roseola are different diseases. Magnification: unknown.