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Poliomyelitis is
caused by a poliovirus. Viruses are
particles of either RNA or
DNA, and
are neither living nor dead. They cannot do anything until they find
an appropriate cell whose internal mechanism they can commandeer to
start reproducing themselves. |
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Karl Landsteiner and Erwin Popper discovered poliovirus in 1908 by proving
that it was not a bacterium that caused the paralysis, but a much smaller
entity—a virus. |
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Viruses are the smallest and simplest infectious agents known … so far. |
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Viruses are not cells, but have an attraction (tropism) for receptors on
certain cells. |
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In addition to polio, RNA viruses include HIV, influenza A, the common
cold, hepatitis A and C, yellow fever, rabies, mumps, and measles. |
Left: Copy of the first image of poliovirus taken with
an electron microscope, 1953
Right: The electron microscope, invented in 1931, allowed researchers to see
viruses for the first time. Since it uses a beam of high-energy electrons (which
have short wave lengths), an electron microscpe is capable of much greateer
magnifcations than a conventional light microscope. Courtesy
of David Sarnoff Library
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Virology
Virology, the study of viruses, started in the 1890s as an offshoot of bacteriology. Viruses were defined primarily by their small size. Size was determined by the virusís ability to pass through a filter such as this porcelain one, and by its invisibility under a light microscope. |
“If the ability to replicate is one of life’s
attributes, then polio is a chemical with a life cycle…. Viruses
are ‘living’ chemicals. They have structural uniformity,
like crystals, but can only self-replicate inside living cells. Poliovirus
is made up of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur:
From these elements, the virus forms its RNA (ribonucleic acid) genes,
and its protective protein coat.”
—Jeronimo Cello, Aniko V. Paul, Eckard Wimmer, creators of the synthesized
poliovirus, 2002 |
Left: Picture of poliovirus. The poliovirus
is extremely small, about 50 nm (nanometer = one-billionth of a meter)
Courtesy of David Belnap and James Hogle
Right: Cross-section of the poliovirus showing the RNA, capsid, and nerve cell
receptors
Illustration courtesy of Link Studio
ìMost viruses are far too tiny to be seen in
an ordinary high-powered microscope, except perhaps as a minute spot
of light against a dark background.î
—Francis Crick, co-discoverer of the double-helix structure of DNA, 1988 |
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Poliomyelitis: Inflammation of the gray matter of
the spinal cord. P., acute anterior, acute inflammation of the anterior
horns of the gray matter of the spinal cord, leading to a destruction
of the large multipolar cells of these horns. It is most common in children,
coming on during the period of the first dentition and producing a paralysis
of certain muscle groups or of an entire limb.
Gould’s Illustrated Medical Dictionary, 1895
Poliomyelitis: An inflammatory process involving the gray matter of
the cord. Acute anterior p., inflammation of the anterior cornua of the
spinal cord; an acute infectious disease caused by the poliomyelitis
virus and marked by fever, pains, and gastroenteric disturbances, followed
by a flaccid paralysis of one or more muscular groups, and later by atrophy.
Stedman’s Medical Dictionary, 1995 |
