Parasites reduce the reproductive output of their hosts, limit their growth, and sometimes even castrate or hill them. Under certain conditions however, a parasitized host may be better off than an uninfected one. Such 'nice' parasites have a ‘pleiotropic' action on their hosts. Parasites can be pleiotropic either in space (in which case they have a beneficial effect on the host in one environment while being detrimental in another) or in time (the parasite is beneficial at one stage of the host's development and ‘costly' at another stage). Such pleiotropic parasites may constitute the intermediate stage between parasitism and mutualism.