Prophage reactivation can occur by recombination between a UV-damaged infecting phage λ chromosome and a homologous phage genome integrated into the bacterial DNA and existing in a prophage state.

How is prophage formed?

Prophages are formed when temperate bacteriophages integrate their DNA into the bacterial chromosome during the lysogenic cycle of the phage infection to bacteria.

What triggers a prophage?

What triggers a prophage to pop back out of the chromosome and enter the lytic cycle? At least in the laboratory, DNA-damaging agents (like UV radiation and chemicals) will trigger most prophages in a population to re-activate.