Mechanically, a strong card, as others have mentioned, and anyone who plays it can attest.
Thematically, it is also a strong card. A virulent Parasite infecting the Corp's ICE? Mmm... Delicious flavor.
Design-wise, it is a curious case of "what were they thinking?" Let me explain.
Most cards with a subtype typically follow a set of rules or principles. Consider current events. All current events have the same 1st line in their text box: "this event is not trashed until an agenda is stolen or another current is played" which makes the current cards distinct from other event types. To give an even more obvious example, the three breaker types all break a specific kind of ice - Killers for Sentries, Decoders for Gates, and Fracters for Barriers. All console cards come with the "console limit" text, and all double events cost an extra click, and the Caissa programs all have a similar "behavior". I could go on and on and on.
But then we come to the virus subtype. What do the virus cards have in common? Virus counters, of course! But wait, no, not all of them. Some, maybe even most, use virus counters, but a few don't follow that rule. Strange. Even more baffling, in my opinion, is that FFG didn't use the "install only on a rezzed piece of ICE" as the defining ability of virus cards - an ability that only makes sense the more you think about it. Why would I be installing viruses in my rig? I wouldn't, but I would be infecting the Corp systems, for sure. Also, how does the Corp wipe viruses off of my PC? Purging viruses leeching off of their ICE is one thing, but did they some how hack my comp and start deleting my viruses? Oh, the confusion.
Parasite presents a "what could have been" that will always make me sigh. I can tolerate broken cards and "useless" cards (fortunately A:NR has very, very few) but design inconsistency kills me. I will continue to play Parasite and all the rest, but I do so with a heavy heart.