phage

A program that modifies other programs or databases in unauthorized ways; esp. one that propagates a virus or Trojan horse. See also worm, mockingbird. The analogy, of course, is with phage viruses in biology.

The noun has one meaning:

Meaning #1: virus parasitic in bacteria; it uses the bacterium's machinery and energy to produce more phage until the bacterium is destroyed and phage is released to invade surrounding bacteria
  Synonym: bacteriophage