Coronaviridae is a family of enveloped, positive-stranded RNA viruses. The viral genome is 26–32 kb in length. Virions are spherical, 120–160 nm across (Coronavirinae), bacilliform, 170–200 by 75–88 nm (Bafinivirus) or found as a mixture of both, with bacilliform particles characteristically bent into crescents (Torovirus). The particles are typically decorated with large (~20 nm), club- or petal-shaped surface projections (the “peplomers” or “spikes”), which in electron micrographs of spherical particles create an image reminiscent of the solar corona. Members of this family are thus referred to as coronaviruses, as are members of the subfamily Coronavirinae. Coronaviruses infect a wide range of mammals and birds and occur worldwide. Although most diseases are mild, sometimes they can cause more severe situations in humans, such as, for example, the infection of the respiratory tract known as Severe Acute Respiratory Syndrome (SARS). They can also cause enteric infections in very young infants and, in rare situations, neurological syndromes.