"one species and one species only"
[transcript: flea, the common name for the order Siphonaptera, includes 2,500 species. end transcript]
you might want to do a little bit of research about the organisms you want to drive to extinction before you claim to know exactly what you're doing, but anyway...
there are already a number of people in the notes bringing up these points, which apply to all species, but here they are again anyway:
firstly, nothing has to have "a purpose", but things don't just evolve for no reason and contribute absolutely nothing to the ecosystem they're in. that's just not how it works.
secondly, parasites are basically the Robin Hood of the ecosystems they're in. they take nutrients from large, long lived organisms, and make it available for smaller organisms, thereby redistributing nutrients that would essentially get hoarded away otherwise.
yes, being bitten is annoying, but we can't just kill off everything that annoys us. in fact, parasites being annoying causes their hosts to leave the area, which affects things like migration routes, and helps prevent areas being overgrazed because the megafauna leave due to the parasites before that can happen.
yes, parasites can spread diseases, but it's much more productive and less harmful to ecosystems to put money and effort towards curing and preventing those diseases than trying to kill off their vectors.
everything in an ecosystem (with the exception of introduced species) has evolved together and adapted to each others' presence. you can't just remove an entire species from that complex, interconnected system and expect there to be no negative repercussions