  Upon infection of their bacterial hosts temperate phages must chose between
lysogenic and lytic developmental strategies. Here we apply the game-theoretic
bet-hedging strategy introduced by Kelly to derive the optimal lysogenic
fraction of the total population of phages as a function of frequency and
intensity of environmental downturns affecting the lytic subpopulation.
"Well-temperate" phage from our title is characterized by the best long-term
population growth rate. We show that it is realized when the lysogenization
frequency is approximately equal to the probability of lytic population
collapse. We further predict the existence of sharp boundaries in system's
environmental, ecological, and biophysical parameters separating the regions
where this temperate strategy is optimal from those dominated by purely
virulent or} dormant (purely lysogenic) strategies. We show that the virulent
strategy works best for phages with large diversity of hosts, and access to
multiple independent environments reachable by diffusion. Conversely,
progressively more temperate or even dormant strategies are favored in the
environments, that are subject to frequent and severe temporal downturns.
