Symbiont. The word you're looking for is symbiont, not parasite. A symbiont lives in harmony with the host, exchanging something in exchange for the resource it consumes (in this case protection). A parasite just takes, and gives nothing back to the host.
(Of course, let's ignore the obvious joke about some AV solutions like Symantec.)
Yeah, the distinction between a symbiont and a parasite is mostly of degree, not kind, and AV software these days is increasingly being found on the blurry line between the two.
(Of course, let's ignore the obvious joke about some AV solutions like Symantec.)