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.