| Many people are exposed to viruses and
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| | determine whether an organism is alive.
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| become sick, but for the majority of the
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| | The two biggest blocks to viruses being
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| population, they have no idea what a
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| | accepted as 'living' organisms is their
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| virus actually is. A virus is something
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| | lack of a cell membrane and their
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| that causes an illness and it is
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| | inability to metabolize on their own.
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| contagious. At its most basic level a
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| | Another criteria for living organisms
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| virus is just a strand of DNA surrounded
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| | that viruses lack is adaptation, or the
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| by a protein shell, which is what makes
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| | ability to change and evolve depending on
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| it different from other fragments of DNA
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| | their environment. Viruses lack any hard
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| such as prions and viroids. A virus
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| | parts that can fossilize such as bones
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| cannot reproduce outside of a host cell
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| | and they are too small to leave imprints
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| and is considered by many scientists as
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| | in sand, like some ancient jellyfish.
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| not being truly 'alive'. Viruses can
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| | Thus there are no fossil records of
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| infect plants and animals and some are
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| | viruses, which makes showing
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| even able to infect bacteria.Humans have
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| | relationships between different viruses
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| dealt with viral diseases, such as rabies
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| | very difficult.Because viruses do not
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| and smallpox, for many centuries. As
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| | leave any fossil remains, the best way to
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| early as the 18th century, people were
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| | hypothesize their origins has been to use
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| observed who had been exposed to smallpox
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| | molecular techniques, that is to study
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| or cowpox, a similar virus and had
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| | the genetic materials of different
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| subsequent immunity to smallpox. In the
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| | viruses and try to determine any
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| late 19th century a porcelain filter,
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| | relationships by finding similarities
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| developed by Charles Chamberland, was
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| | between them. There are two main
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| used to indirectly study the tobacco
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| | hypotheses concerning virus origins. The
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| mosaic virus. Dmitri Ivanowski performed
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| | first hypothesis, involving the small
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| experiments on crushed tobacco leaf
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| | viruses with only a few genes, is that
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| extracts that showed infected plants were
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| | these small viruses may be runaway
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| still infectious after bacteria were
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| | snippets of a living organisms DNA.
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| filtered out. Many others performed
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| | These small DNA fragments could have come
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| experiments that showed similar
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| | from plasmids or transposons, which are
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| filterable disease causing agents. Felix
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| | transferable genetic elements and are
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| d'Herelle and Frederick Twort, working
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| | prone to entering or exiting genomes.
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| independently, found that bacteria could
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| | The second hypothesis involves the larger
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| be infected by viruses, not just animals
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| | viruses. These large viruses may have
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| and plants. In the 1930s, Wendell
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| | been small parasitic cells at one time
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| Stanley showed that the tobacco mosaic
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| | and, through the process of
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| virus was mostly protein and shortly
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| | reverse-evolution, lost any genes not
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| after, it was further separated into
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| | needed by their parasitic lifestyle.
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| genetic material and a protein shell.A
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| | There are a few bacteria, Rickettsia and
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| virus is not considered truly alive
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| | Chlamydia, that support this hypothesis
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| because it is missing several of the main
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| | because they cannot reproduce outside of
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| qualities that scientists use to
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| | a host cell, like viruses.
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