Vertical transmission favors prudent strategies whereas competition between parasite strains and horizontal transmission favor virulence. The evolution of parasites is not independent of the evolution of their hosts, who defend themselves. Generally, parasites have an advantage in this arms race due to their shorter generation time and greater numbers. Co-evolution leads to extensive local adaptation. That is, parasites in a particular area infect hosts from that area more efficiently than they infect hosts from another geographically distinct population.
Local adaptation is a general phenomenon found in most host-parasite relationships.
Strategies are often based on mimicry. Cuckoo eggs, for example are practically identical to the eggs of the host species. However, some hosts have a remarkable ability to distinguish their own eggs from the intruder's. Such adaptations and counter-adaptations present an ideal scenario for an evolutionary arms race. Evidence from a cross-inoculation experiment.