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.