Colored transmission electron micrograph of coronavirus OC43. Pleomorphic virions average 100 nm in diameter and are covered with club-shaped knobs, which suggest a halo or crown, hence the name coronavirus. Coronaviruses usually produce an afebrile cold in adults, characterized by nasal discharge, and malaise; infections can also be associated with pneumonia and pleural reactions. These viruses are also implicated in gastroenteritis. Immunity is serotype specific; antigenic heterogeneity allows for multiple symptomatic reinfections. Worldwide these viruses are a major cause of respiratory disease between late fall and early winter, accounting for 10-30% of all colds. Coronaviruses were first isolated in 1965. Their genome consists of single-stranded, linear, non-segmented, positive-sense RNA.