The type of interaction between two organisms of different species in which one is benefited from its partner and another organism is neither harmed nor benefited is called commensalism. An association between the shark and the sucker fish is an example of commensalism. Here, the sucker fish gets the benefit (food and protection) from the shark while the shark does not get benefited or be harmed.

Parasitism is the relationship between two living organisms of different species, in which one organism called a parasite obtains its food directly from another living organism called the host. From this interaction, the parasite is benefited, but the host is harmed. The parasite is smaller compared to the host. Ascaris worm living in the intestine of human beings is an example of parasitic interaction, where the worm lives inside the human intestine and develops into an adult. The growth of the worm causes adverse effects on the host such as nausea, vomiting, diarrhoea, loss of appetite, worms in stool, etc. Some species of this worm cause the disease ascariasis.