In the figure, each red, white, yellow and green squiggle is a shorthand representation of a large protein molecule. 96 copies of each kind of protein lock together to make the nearly spherical virus particle. Inside, in the form of RNA, is the genetic code that carries all of the information the virus needs to replicate itself.

It's appropriate to begin out thinking about living things with viruses, because they challenge our views about what it means to be alive.

Viruses are capable of replicating themselves by infecting the cells of other organisms and sometimes co-opting the machinery they find there to make copies of themselves.

Viruses undergo the random mutations that lead to natural selection and long-term changes in their appearance and function. Viruses do not, however, "eat." They don't take in food, convert it to energy and create waste products. Many viruses can stay dormant for a long time, then reproduce when a host cell is encountered. You might say that, compared to a plant or an animal, viruses are about "half alive."

These biology sections explore the main characteristics and functions of living things.