A community perspective on the evolution of virulence

Studies of the evolution of virulence aim at understanding how and why certain parasite strains have evolved to cause morbidity and mortality to their hosts, while others have remained benign. In parasitism, according to definition, a parasite benefits at the expense of its host. These benefits are usually realized upon harming the host following infection. Conventional wisdom holds that damaging the host is detrimental to the interests of the invading parasites. Therefore, parasites should have evolved to become avirulent to their hosts as they otherwise risk driving their hosts, and therefore themselves, to extinction. This view, which has been criticized for its reliance on group selection, no longer prevails (Lenski & May, 1994). Instead, the evolutionary theory of virulence strives to elucidate the underlying selective forces that lead to increased or reduced virulence by examining the costs and benefits of virulence to both parasite and host. Ultimately, this theory attempts to identify which expressions of virulence are adaptive in the long-run, and which are non-adaptive, i.e. coincidental or short-sighted (Levin & Bull, 1994; Levin, 1996).