Virus – an invisible, potent, deadly little agent of disease that commandeers living cells and forces them to make more of itself. Viruses can cause a wide range of ailments, from common colds and flu to deadly pneumonia and Ebola. They can explode into pandemics that kill tens of thousands, upend economies, and sweep the globe. And they’re remarkably simple, with just a few thousand genes compared to the 20,000 in each human cell.
A virus is a package of genetic information protected by a protein shell for delivery into a host cell where it can be expressed and replicated. Viruses are not alive, but they have some properties of life and, like seedlings, can remain inert for years while waiting to be planted (see Fig. 2.1).
The virus shell, called the capsid, protects the nucleocapsid containing the viral genetic material from physical, chemical and enzymatic damage. It also has sites that bind to receptor molecules on the surface of the target host cell, much as a key fits into a lock. This specificity explains why viruses are usually only found in certain types of cells, such as those in the respiratory tract or liver.
The host cell then takes over the viral genetic material and starts making more viruses, diverting its normal metabolism to the process. Eventually, enough viruses are produced to overwhelm the host, and they break out of the cell, infecting other cells, and potentially the entire organism.
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