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204 PART 5 Antiviral Drugs
The understanding of how they interact with and
shape the existence of living cells, however, has
increased greatly since they were described by
Louis Pasteur in the late nineteenth century.
An in-depth discussion of the structure of
viruses is beyond the scope of this text, but a basic
understanding of viruses will help you understand
the actions of antiviral drugs. Viruses are highly
diverse, though nearly all of them share a few common
characteristics. Many are covered by a viral
envelope as their outmost layer, composed of elements
of the host cell membrane, endoplasmic
reticulum, or nuclear envelope. This layer covers
the capsid, a shell composed of identical building
blocks of capsomeres. The capsid protects the viral
nucleic acid, which is either DNA or RNA but not
both (as in cells). The DNA or RNA can be either
single- or double-stranded. Finally, many viruses
contain enzymes that catalyze reactions that lead
to their replication or cell entry. Viruses cannot
synthesize their own components to replicate—
they are dependent on host cellular processes for
all synthetic functions. Individual complete particles
of virus are termed virions.
The specific steps of the viral life cycle differ
from virus to virus, but they follow the same
basic pathway. Viruses spread from host to host
through various means, some through direct inhalation,
some through direct fluid exchange, some
through vectors such as mosquitoes. Once a virus
reaches its target cell, it has to penetrate the cell
membrane. Specific receptors on the cell and viral
surfaces often facilitate this process. The virus
then uncoats and releases its genetic information
from the capsule into the host cell. The host cell