How A Single Parasite Species Can Change Every Life-Form Around It

Parasites are nature's freeloaders, living off their hosts while giving nothing in return. But scientists have come to appreciate that even the greediest parasites can indirectly benefit other species by manipulating ecosystems - providing food, assisting predators and even building habitats.
But how could organisms as small and selfish as parasites have a similarly significant impact on their surrounding environments? Frequently, they do so through the manipulation of other, larger species.