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A virus is a small particle which can infect other biological organisms. The
term "virus" usually refers to those particles which infect eukaryotes
(multi-celled organisms and many single-celled organisms), whilst
"bacteriophage" or "phage" is used to describe those infecting prokaryotes
(bacteria and bacteria-like organisms). Typically these particles carry a
small amount of nucleic acid (either DNA or RNA) surrounded by some form of
protective "coat" consisting of protein, or protein and lipid.
One of several possible viral classifications:
* Class I double stranded DNA.
* Class II single stranded DNA.
* Class III double stranded RNA.
* Class IV positive single stranded RNA itself acting as mRNA.
* Class V negative single stranded RNA used as a template for mRNA
* Class VI positive single stranded RNA with a DNA intermediate
* Class VII double stranded DNA with an RNA intermediate in replication.
The protective coat normally also enables the infective process which can
occur by a variety of different mechanisms. The practical upshot of all of
these is that the host cell's replication machinery is hijacked to create
more of the virus particles, hence completing the life cycle. Viruses are
somewhere between being living and non-living. They can reproduce and show
inheritance, but are reliant on the complex enzymes of their hosts, and in
many ways can be treated like ordinary molecules (for instance, they can be
crystalized). Whether or not they are "alive," it is clear that they are
obligate parasites, and have no form which can reproduce independent of
their host. Like most parasites they have a specific host range, sometimes
specific to one species (or even limited cell types of one species) and
sometimes more general.
Examples of diseases caused by viruses include the common cold, which is
caused by a variety of related viruses; smallpox; AIDS, which is caused by
HIV; and cold sores, which are caused by herpes simplex. Recently it has
been shown that cervical cancer is caused at least partly by papillomavirus
(which causes papillomas, or warts), representing the first significant
evidence in humans for a link between cancer and an infective agent.
Because they use the machinery of their host cells, viruses are difficult to
kill. The most effective medical approaches to viral diseases, thus far, are
vaccination to prevent infection, and drugs that treat the symptoms of viral
infections. Patients often ask for antibiotics, which are useless against
viruses, and their misuse against viral infections is one of the causes of
antibiotic resistance in bacteria. That said, sometimes the prudent course
of action is to begin a course of antibiotic treatment while waiting for
test results to determine whether the patient's symptoms are caused by a
virus or a bacterial infection.
The origin of viruses is not entirely clear, but the currently favoured
explanation is that they are derived from their host organisms, originating
from transferrable elements like plasmids or transposons. It has also been
suggested that they may represent extremely reduced microbes, appeared
separately in the primordial soup that gave rise to the first cells, or that
the different sorts of viruses appeared through different mechanisms.
Other infectious particles which are even simpler in structure than viruses
include viroids, virusoids, and prions.
The word comes from the Latin virus, referring to poison and other noxious
things. Today it is used to describe the biological viruses discussed above
and also as a metaphor for other parasitically-reproducing things, such as
ideas. The term computer virus has become another well-defined sense of the
word. The word virion is used to refer to a single infective viral particle.
Despite frequent claims to the contrary, the only correct English plural of
the word for any of these senses is viruses. The Latin word does not appear
to have had a plural. Virii would be the plural of the word virius, and viri
was the plural of the word vir, meaning man. See  for more on this.