What are the best parasite species competitive viability? Rank them all with this parasite species competitive viability tier list maker. Drag your favorites into S tier and drop the worst into D tier. Share your parasite species competitive viability tier list on social media, compare rankings with friends, and see what other Americans think. The ultimate way to rank and debate parasite species competitive viability online. Popular categories include parasites, diseases, us-health, medical-conditions.