Antiviral drugs are a class of
medication used specifically for
treating viral infections.
Like antibiotics for bacteria, specific
antivirals are used for specific
viruses. Unlike antibiotics, antiviral
drugs do not destroy their target
pathogen; instead they inhibit their
Antiviral drugs are one class of
antimicrobials, a larger group which
also includes antibiotic, antifungal and
antiparasitic drugs. They are relatively
harmless to the host, and therefore can
be used to treat infections. They should
be distinguished from viricides, which
are not medication but destroy virus
particles outside the body.
Most of the antivirals now available are
designed to help deal with HIV, herpes
viruses (best known for causing cold
sores and genital herpes, but actually
causing a wide range of diseases), the
hepatitis B and C viruses, which can
cause liver cancer, and influenza A and
B viruses. Researchers are working to
extend the range of antivirals to other
families of pathogens.
Designing safe and effective antiviral
drugs is difficult, because viruses use
the host's cells to replicate. This
makes it difficult to find targets for
the drug that would interfere with the
virus without also harming the host
The emergence of antivirals is the
product of a greatly expanded knowledge
of the genetic and molecular function of
organisms, allowing biomedical
researchers to understand the structure
and function of viruses, major advances
in the techniques for finding new drugs,
and the intense pressure placed on the
medical profession to deal with the
human immunodeficiency virus (HIV), the
cause of the deadly acquired
immunodeficiency syndrome (AIDS)
Through the mid- to late-20th century,
medical science and practice included an
array of effective tools, ranging from
antiseptics to vaccines and antibiotics,
but no drugs to treat viral infections.
While vaccines were effective in
preventing many viral diseases, they
could not help once a viral infection
set in. Prior to the development of
antivirals, when someone contracted a
virus, there was little that could be
done other than treating the symptoms
and waiting for the disease to run its
The first experimental antivirals were
developed in the 1960s, mostly to deal
with herpes viruses, and were found
using traditional trial-and-error drug
discovery methods. Researchers grew
cultures of cells and infected them with
the target virus. They then introduced
chemicals into the cultures they thought
were likely to inhibit viral activity,
and observed whether the level of virus
in the cultures rose or fell. Chemicals
that seemed to have an effect were
selected for closer study.
This was a very time-consuming,
hit-or-miss procedure, and in the
absence of a good knowledge of how the
target virus worked, it was not
efficient in discovering antivirals that
were effective and had few side effects.
It was not until the 1980s, when the
full genetic sequences of viruses began
to be unraveled, that researchers began
to learn how viruses worked in detail,
and exactly what chemicals were needed
to thwart their reproductive cycle.
Dozens of antiviral treatments are now
available, and medical research is
rapidly exploiting new knowledge and
technology to develop more.
 Virus life cycle
Viruses consist of a genome and
sometimes a few enzymes stored in a
capsule made of protein (called a capsid),
and sometimes covered with a lipid layer
(sometimes called an 'envelope').
Viruses cannot reproduce on their own,
so they propagate by subjugating a host
cell to produce copies of themselves,
thus producing the next generation.
Researchers working on such "rational
drug design" strategies for developing
antivirals have tried to attack viruses
at every stage of their life cycles.
Some species of mushrooms have been
found to contain multiple antiviral
chemicals with similar synergistic
effects. Viral life cycles vary in
their precise details depending on the
species of virus, but they all share a
* Attachment to a host cell.
* Release of viral genes and possibly
enzymes into the host cell.
* Replication of viral components using
* Assembly of viral components into
complete viral particles.
* Release of viral particles to infect
new host cells.