lambda phage

bacteriophage \bacteriophage\ n. sing. & pl.
   a virus which infects bacteria; -- also colloquially called
   {phage} in laboratory jargon.

   Note: Bacteriophages are of many varieties, generally
         specific for one or a narrow range of bacterial
         species, and almost every bacterium is susceptible to
         at least one bacteriophage. They may have DNA or RNA as
         their genetic component. Certain types of
         bacteriophage, called

   {temperate bacteriophage}, may infect but not kill their host
      bacteria, residing in and replicating either as a plasmid
      or integrated into the host genome. Under certain
      conditions, a resident temperate phage may become induced
      to multiply rapidly and vegetatively, killing and lysing
      its host bacterium, and producing multiple progeny. The
      {lambda phage} of {Eschericia coli}, much studied in
      biochemical and genetic research, is of the temperate
      type.
      [PJC] bacteriophagic