Published: October 2007
Did You Know?
In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.
What is a virus?

Generally, a living thing is defined as a cellular organism capable of independent metabolic processes and replication. A virus, however, consists of genetic material enclosed by a protein coat and is unable to replicate on its own. Think of a virus as a book of instructions with no one to read the book and start building. It comes to "life" when it invades a host cell and hijacks its machinery, forcing the cell to read and replicate the virus many times. Eventually, the copies of the virus burst forth from the cell, seeking out new hosts.

–Emily Krieger