tapeworm vs bacteria, parasite vs parasite, symbiotic or otherwise. and you have plenty of other parasites.
edit: maybe a simpler example. Try to describe why tapeworms are different from bacteria, without using "multicellular". both can cause harm. both eat your food. both can result in weight loss.
Bacteria that aid in weight control don't just "eat your food," they actually help your body break down food correctly. That's why you take a pro-biotic after going through a round of anti-biotics - you need to re-grow your gut flora to aid in proper digestion.
edit: maybe a simpler example. Try to describe why tapeworms are different from bacteria, without using "multicellular". both can cause harm. both eat your food. both can result in weight loss.