This paper addresses the problem of selecting route to a given destination on a load map under a dynamic environment. The proposed solution is using a genetic algorithm adopting viral infection. The method is to use viruses as domain specific knowledge. A part of an arterial road is regarded as a virus. We generate a population of viruses in addition to a population of routes. Crossover and infection determine the optimal combination of viruses. When traffic congestion changes during driving, an alternative route can be generated using viruses and other routes in the population in the shortest time. Experiments using actual road maps show the infection is effective for the problem.