Parasitism is not limited to the human world - as some might imagine - but it also exists in the animal world, and in the plant world as well. Many plants cannot obtain their own food, so they cannot, for the sake of their lives, take for themselves a host on which they live, parasitizing. Examples: There are many examples of this, as the dodder parasitizes clover, the halo parasitizes beans, and wheat rust parasitizes it. I wish the matter would stop at this point. In fact, some fungi parasitize on humans themselves, and that is the fungus that causes common squash disease.
In the animal world, parasitism is more evident than in the plant world, as parasitic diseases have become common among a large number of citizens, and these diseases result from human infection with parasites.