Coronaviruses contain a single-stranded, positive-sense RNA genome, which is sur-rounded by an envelope that includes a lipid bilayer derived from intracellular rough en-doplasmic reticulum and Golgi membranes of infected cells. Petal- or club-shaped spikes (peplomers) measuring approximately 13 nm project from the surface of the envelope, giving the appearance of a crown of thorns or a solar corona. The peplomers play an im-portant role in inducing neutralizing and cellular immune responses. Like the rhi-noviruses, coronaviruses are considered primary causes of the common cold. Based on serologic studies, it is estimated that they may cause as many as 5 to 10% of common colds in adults and a similar proportion of lower respiratory illnesses in children.
The number of serotypes is unknown. Two strains (229E and OC43) have been stud-ied to some extent; it is clear that they can cause outbreaks similar to those of the rhi-noviruses and that reinfection with the same serotype can occur. The cellular receptors for these strains are a cell surface metalloprotease and a sialic acid receptor similar to that bound by influenza C virus.
In late 2002, an illness called severe acute respiratory syndrome (SARS) appeared in China, spread throughout Asia, and is now found worldwide. The etiology has been iden-tified as a previously undescribed coronavirus, with unusually high virulence for humans.