The condition caused by a parasite living off another organism:

I believe the condition is infestation. Infestation is the state of being invaded or overrun by pests or parasites. It can also refer to the actual organisms living on or within a host. For example being invaded by ectoparasites such as lice, mites, or ticks. It may also refer to the presence of an unusually large number of insects or animals in a place, typically so as to cause damage or disease.