In
virology,
temperate refers to the ability of some
bacteriophages (notably
coliphage λ) to display a
lysogenic life cycle. Many (but not all) temperate phages can integrate their
genomes into their host
bacterium's chromosome, together becoming a
lysogen as the phage genome becomes a
prophage. A temperate phage is also able to undergo a productive, typically
lytic life cycle, where the
prophage is expressed, replicates the phage genome, and produces phage progeny, which then leave the
bacterium. With phage the term
virulent is often used as an antonym to temperate, but more strictly a virulent phage is one that has lost its ability to display lysogeny through mutation rather than a phage lineage with no genetic potential to ever display lysogeny (which more properly would be described as an obligately lytic phage).