A prophage is a bacteriophage genome that has inserted itself, non-harmfully, into the bacterial chromosome during its lysogenic cycle. Upon detection of host cell damage, the prophage is excised from the genome and activated into the lytic (proliferative) cycle by a process called induction. Prophages are an important agent of horizontal gene transfer (transfer of DNA between individuals who are not parent and offspring), sometimes causing the spread of favourable genes among a bacterial population by the process of transduction. Prophages can modify the phenotype of their host, causing them to develop pathogenicity by synthesising new toxins or giving them immune resistance.