the.com/parasitism
A relationship where one party invoices the other in flesh and pays nothing.
means An interaction where one organism lives on or in a host, gaining at the host's expense.
from From Greek parasitos, one who eats beside another — originally a guest who flattered for a free meal, before biology made it literal.
HeadcountParasites may outnumber free-living species on Earth.
Mind controlSome parasites rewire host behavior to spread themselves.
Russian dollsParasites have parasites, called hyperparasites.