Virus
Year of Discovery: 1898
What Is It? The smallest, simplest living organism and causative agent for
many human diseases, from simple colds to deadly yellow fever.
Who Discovered It? Dmitri Ivanovsky and Martinus Beijerinick
Why Is This One of the 100 Greatest?
Far smaller than cells and bacteria, viruses are the smallest life forms on Earth—so
small they can only reproduce inside some host cell and do it by taking over control of that
cell. Viruses are so small they easily slip through virtually any filter or trap. Their discovery
answered many medical questions at the beginning of the twentieth century and completed
Pasteur’s germ theory.
Viruses cause many of the most dangerous human diseases. Until they were discovered, medical science had ground to a halt in its advance on curing these human illnesses.
When Beijerinick discovered viruses, he actually discovered a new life form, one too small
to be seen with any microscope other than a mighty electron microscope.
How Was It Discovered?
French scientist Louis Pasteur discovered germs (microscopic bacteria) and claimed
that germs caused disease and rot. However, he was never able to find a microorganism
(germs) that caused rabies, though he tried for over a decade before giving up in 1885. It left
a shadow of doubt over his germ theory.
Another disease for which no one could find an identifiable causative agent was tobacco mosaic disease (so called because a mosaic pattern forms on the leaves of infected
plants). In 1892 Russian botanist Dmitri Ivanovsky decided to search for this mysterious
agent. (It was safer to work with tobacco mosaic disease than with deadly rabies.)
Ivanovsky mashed up infected leaves and passed the fluid through various paper and ceramic filters. These filters were supposed to trap all organisms—even the tiniest bacteria.
However, the fluid that strained through these sets of filters could still infect healthy
tobacco plants with mosaic disease. That meant that Ivanovsky hadn’t trapped the causative
agent in his filters. He tried different filter materials, different treatments, and baths for the
leaves and mashed juice. His results were always the same. Whatever caused this disease,
Ivanovsky couldn’t trap it in a filter.
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