Life Cycle of a Virus
Computer viruses have a life cycle that starts when they’re created and ends when they’re completely eradicated. The following outline describes each stage.

Creation
Until a few years ago, creating a virus required knowledge of a computer programming language. Today anyone with even a little programming knowledge can create a virus. Usually, though, viruses are created by misguided individuals who wish to cause widespread, random damage to computers.

Replication
Viruses replicate by nature. A well-designed virus will replicate for a long time before it activates, which allows it plenty of time to spread.

Activation
Viruses that have damage routines will activate when certain conditions are met, for example, on a certain date or when a particular action is taken by the user. Viruses without damage routines don’t activate, instead causing damage by stealing storage space.

Discovery
This phase doesn’t always come after activation, but it usually does. When a virus is detected and isolated, it is sent to the International Computer Security Association in Washington, D.C., to be documented and distributed to antivirus developers. Discovery normally takes place at least a year before the virus might have become a threat to the computing community.

Assimilation
At this point, antivirus developers modify their software so that it can detect the new virus. This can take anywhere from one day to six months, depending on the developer and the virus type.

Eradication
If enough users install up-to-date virus protection software, any virus can be wiped out. So far no viruses have disappeared completely, but some have long ceased to be a major threat.