Viruses for microorganisms are called phages.  Those for bacteria are bacteriophages.  A virus can be thought of as genetic elements within a protein coating.  When injected into a cell, this genetic material may alter the host so that its energy and metabolism are directed entirely toward making more virus particles, and ultimately the cell will die.  Some phages are always actively seeking cells in which to reproduce.  Other phages can attach to the bacterial chromosome and remain passive for a long time.  Active phages are called virulent.  Those that can be passive are called temperate or lysogenic; they may become virulent if the system is shocked by an event such as a temperature or pH change, but some host cells survive and continue to multiply.  When not in their virulent stage, temperate phages have no discernable effect on their hosts.

T4 is one of many virulent phagesthat infects E. coli. Its sheath fastens to the cell wall, and constriction forces some phage DNA (some phages are RNA) into the cell to make it into a factory for making more phage particles.