Transmission Electron Micrograph (TEM) of the Zika virus, a member of the family Flaviviridae. Virus particles are 40 nm in diameter, with an outer envelope and an inner dense core. Zika virus is spread to people through mosquito bites. The most common symptoms of Zika virus disease are fever, rash, joint paint, and conjunctivitis (red eyes). The illness is usually mild, with symptoms lasting from several days to a week. Severe disease requiring hospitalization is uncommon.