In an episode of the classic 1950s television comedy The  Honeymooners, 
Brooklyn bus driver Ralph Kramden loudly explains to his  wife, Alice, “You know 
that I know how easy you get the virus.” Half a century  ago even regular folks 
like the Kramdens had some knowledge of viruses—as  microscopic bringers of 
disease. Yet it is almost certain that they did not know  exactly what a virus 
was. They were, and are, not alone. 
For about 100 years, the scientifi c community has repeatedly changed its  
collective mind over what viruses are. First seen as poisons, then as  
life-forms, then biological chemicals, viruses today are thought of as being in  a gray 
area between living and nonliving: they cannot replicate on their own but  
can do so in truly living cells and can also affect the behavior of their hosts  
profoundly. The categorization of viruses as nonliving during much of the 
modern  era of biological science has had an unintended consequence: it has led 
most  researchers to ignore viruses in the study of _evolution_ 
(http://www.sciam.com/topic.cfm?id=evolution) . Finally, however, scientists are beginning  
to appreciate viruses as fundamental players in the history of life. 
Coming to Terms 
It is easy to see why viruses have been  diffi cult to pigeonhole. They seem 
to vary with each lens applied to examine  them. The initial interest in 
viruses stemmed from their association with  diseases—the word “virus” has its 
roots in the Latin term for “poison.” In the  late 19th century researchers 
realized that certain diseases, including rabies  and foot-and-mouth, were caused 
by particles that seemed to behave like _bacteria_ 
(http://www.sciam.com/topic.cfm?id=bacteria)  but were much smaller. Because they were  clearly 
biological themselves and could be spread from one victim to another  with obvious 
biological effects, viruses were then thought to be the simplest of  all living, 
gene-bearing life-forms. 
Their demotion to inert chemicals came after 1935, when Wendell M. Stanley  
and his colleagues, at what is now the Rockefeller University in New York City, 
 crystallized a virus— tobacco mosaic virus—for the fi rst time. They saw 
that it  consisted of a package of complex biochemicals. But it lacked essential 
systems  necessary for metabolic functions, the biochemical activity of life. 
Stanley  shared the 1946 Nobel Prize— in chemistry, not in physiology or 
medicine—for  this work. 
Further research by Stanley and others established that a virus consists of  
nucleic acids (DNA or RNA) enclosed in a protein coat that may also shelter  
viral proteins involved in infection. By that description, a virus seems more  
like a chemistry set than an organism. But when a virus enters a cell (called 
a  host after infection), it is far from inactive. It sheds its coat, bares 
its  genes and induces the cell’s own replication machinery to reproduce the  
intruder’s DNA or RNA and manufacture more viral protein based on the  
instructions in the viral nucleic acid. The newly created viral bits assemble  and, 
voilà, more virus arises, which also may infect other cells. 
These behaviors are what led many to think of viruses as existing at the  
border between chemistry and life. More poetically, virologists Marc H. V. van  
Regenmortel of the University of Strasbourg in France and Brian W. J. Mahy of  
the Centers for Disease Control and Prevention have recently said that with  
their dependence on host cells, viruses lead “a kind of borrowed life.”  
Interestingly, even though biologists long favored the view that viruses were  mere 
boxes of chemicals, they took advantage of viral activity in host cells to  
determine how nucleic acids code for proteins: indeed, modern _molecular 
biology_ (http://www.sciam.com/topic.cfm?id=molecular-biology)  rests on a 
foundation of information  gained through viruses.



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