Following infection of a bacterial cell by a temperate bacteriophage, the phage can develop in one of two ways; lytic development where phage DNA is replicated and more phage particles are assembled, lysing the cell to release new viruses into environment; or lysogenic development where the phage DNA integrates into the bacterial chromosome, where it passively resides through further generations. The stable lysogenic state can quickly flipped to the alternative lytic state if the phage senses that its host is in danger. Thus this simple system serves as a tractable model for biological switches in general. Development in higher organisms can be considered as a series of nested switches, the particular state of each successive switch dictating which developmental pathway will be followed.
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