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The concept of viruses can inspire a sense of mystery and awe.
At times we assign to them imaginary powers that they do not
have, and yet, at other times, they seem to have powers beyond
imagination. Despite success in eradicating viral diseases like
smallpox and polio, humans continue to experience outbreaks of new
viral infections every few months. In fact, viruses are the most prominent
emerging microbes, accounting for over 50% of new outbreaks of
infectious diseases worldwide.
We need no more powerful reminder of these realities than to
consider the sudden onset of a new disease SARS (severe acute respiratory
syndrome) in late 2002. The disease began in mainland China
and soon spread worldwide, causing great fear and panic. In several
countries, infected people were quarantined and tested at travel terminals
for signs of infection (see chapter-opening photo). The virus
was rapidly identified as a new coronavirus, but its origins are yet to be
determined.
A different virus—the West Nile virus—has also caused a great
deal of concern in the United States. Several hundred cases have been
reported, with a number of deaths. This virus has somehow migrated
from Africa or Asia and become established in American birds and
mosquitoes. At about the same time, the Nipah virus jumped hosts from
bats to pigs to humans, causing a deadly epidemic among agricultural
workers in parts of Asia.
Viruses are often blamed for unexplained illnesses and symptoms,
and are usually considered the culprit when other infectious
agents have been ruled out. They are tied intimately to their host cells,
where they often linger and even become part of their host's genetic
material. Because of their roles in disease and genetics, it is very important
to have a working knowledge of the basic characteristics of
viruses. The primary aim of this chapter is to familiarize you with their
many unique properties and to provide a survey of their structure, physiology,
multiplication, and diversity.
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