Their sole purpose is to reproduce themselves. A typical bacteria program does nothing more than reproduce itself exponentially, eventually eating up all the processor capacity, memory, or disk space, denying the user access to those resources. This kind of programming attack is one of the oldest forms of programmed threat. A logic bomb is a program or a section of code built into a program that lies dormant until a predefined condition is met. When that condition occurs, the bomb goes off with a result that is neither expected nor desired.
Time bombs explode frequently.
Depending on who authored the bomb and how many generations of backups it contaminated, the recovery effort ranges from mildly inconvenient to nearly impossible. A password catcher is a program that mimics the actions of a normal sign-on screen but stores supplied ID and password combinations, usually in a hidden file accessible only to the author. Running for three or four days will generally yield more than 90 percent of a site's active system users. Truly clever ones cause no perceptible delay and force no repeated sign-ons. A repeat dialer is a program that continually calls the same number, thus placing it virtually out of service for any other caller.