01.28.09. When lambda phage (a virus) infects a host cell, it either undergoes lysis, in which it exploits cell resources to make copies of itself and kills the cell, or lysogenesis, in which it incorporates its DNA into the cell's and awaits replication (see, for example, this animation from McGraw-Hill). This decision is a probabilistic one, like flipping a weighted coin. Students made probability trees detailing the possible outcomes and their probabilities after several such decisions for a virus and a host cell.
