Parasitism!

 The symbiotic relationship in which one species (parasite) lives in or on the other species (host) for its survival and in return causes harm. The study of parasites is called parasitology. 

Types of parasites:

Ectoparasites 

The parasites living outside the body of host are ectoparasites e.g ticks and mites. 

Endoparasites 

The parasites living inside the hosts body are endoparasites e.g amoeba.

Obligatory parasites

These parasites can not live without their host e.g malarial parasite. 

Facultative parasites

They can complete their life cycle both as free living and parasites e.g certain fungi and brain eating amoeba.