In both predation and parasitism, one species has a positive effect on the second species, but the second species has a negative effect on the first. Examples are predators and parasites. Antelope are good for cheetahs, but cheetahs are bad for antelope. Ditto a dog and its fleas.
The antelope-cheetah interaction is an example of predation. The dog-flea interaction is an example of parasitism. Whether the system is one or the other depends on whether the parasitic organism wants to eat all of you or some of you. In some parasitic interactions — such as deadly parasitic diseases, to which you succumb in the end — there's not much fitness difference being eaten by a cheetah or wasting away from a disease. You're dead either way. You can read about co-evolving disease organisms in "Diseased Systems: Parasitic Co-evolution," later in this chapter.
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