Question

They are always harmful but never beneficial or neutral.

Answer 1

Answer: parasitism

Step-by-step explanation:

parasitism is a symbiotic relationship between 2 organisms, where one organism (the parasite), lives on/in another organism (the host), causing it some harm.

Answer 2

Are you referring to symbiotic relationships between organism? If so, the answer would be parasitism.

One organism, a parasite, takes advantage of another organism at the expense of the host organism.

Ex. A tick bites a dog for blood to use as food. The dog gets an uncomfortable bug bite and the dog might catch a tick borne disease.