Ruminant herbivores gain from the microflora in their gut through digestion of cellulose and in return, the microflora gets the nutrients from the ruminants. The shark doesn't benefit from the remora but the remora gets a free ride from the shark and it feeds on the leftovers from the shark's meal. The human host loses nutrients because the tapeworm gets to eat the food provided by the host. Cats provide shelter and food for the flea but the cat becomes anemic because of blood loss from its parasites.
