In biology/ecology or zoology, parasites are a non-mutual symbiotic relationships between species, where one species, the parasite, benefits at the expense of the other, the host.  This behaviour is known also as parasitism. 

Traditionally parasite (in biological usage) referred primarily to organisms visible to the naked eye, or macroparasites (such as helminths). 


Schistosoma mansoni is an endoparasite that lives in human blood vessels. image: wikipedia 

Parasites can include microparasites, which are typically smaller, such as protozoa (malaria), viruses, and bacteria.  Examples of parasites include the plants mistletoe and cuscuta, and animals such as hookworms.

The study of parasites is known as parasitology and includes investigating the molecular biology, biochemistry of parasites.