Increasingly often teachers will force students to work in groups, which usually results in the smartest person in the group pulling all of the weight while the others more or less freeload. It's pretty easy to describe the others as parasites and the smart person as parasitized, although we might not judge the parasites so harshly since they didn't intend for the class to split up into groups -- they were compelled by the teacher -- so it may be more like a case of stronger people who cannibalize the weakest person when all are forced by shipwreck into a foodless situation.
However, lots of other study groups form voluntarily, with both the smart and the less-smart finding it agreeable. The smart one is obviously providing his smarts -- probably he'll end up doing the other person's homework -- so clearly the less-smart person gets something nice out of the deal. And the smart guy must be getting something good too from the less-smart partner, or else he wouldn't have signed on in the first place. When we see these voluntary exchanges, we should suspect that each person is getting more than they're giving up, unlike the earlier case of mandated partnerships.
If we reflect on what we've seen ourselves or have seen time and again in fiction, we find two recurring patterns of the smart person who agrees to do someone else's homework. One is where the less-smart person is a high-status guy, maybe a good-looking rebel or a popular athlete, who can afford the nerd some protection, teach him how to behave around girls, or have some of his higher status rub off on the nerd when news gets out that they're hanging out with each other, even just to study. The smart guy, of course, won't see his status rise to the heights of the guy whose homework he's doing -- but some improvement is better than none, and again if the smart one agrees to it, the boost to his status must be worth doing the other guy's homework.
The other case is similar: there's some good-looking girl, possibly popular too, who he'd be delighted to be around -- and not just as though he were standing behind her in the cafeteria line, but alone in one of their rooms, close to her, with a frequent verbal back-and-forth, sustained eye contact, and so on. He'll soon realize that he won't get to make out with her, but this lower level of relationship may be worth enough to him, or perhaps he'll be able to attract some female attention after the word gets around that they're hanging out. He may have been invisible to girls before, whereas now at least a couple of 4s or 5s will take notice and he'll have a positive, rather than zero, chance of getting laid. These benefits make it worth giving up his time to do her homework.
And clearly the popular guy or cute girl find the cost of hanging out with the geek worth it, since they'll benefit from passing rather than failing the course, or else they wouldn't have bothered to form the study group to begin with.
Thus, both of them gain from this "social trade," and so it's not quite right to describe one as the parasite and the other as the parasitized. But if we had to point to who's only a little better off, and who's really better off, which would it be? Well, we just look at who's more eager to form this kind of relationship. We can use eagerness as a proxy for what price the person perceives the relationship to cost -- if you're really eager to form the relationship, it's as though you were demanding a lot. Say, you'd be happy to study 40 hours a week. If you're only somewhat eager, you're demanding less, like only 10 hours a week.
Assuming the relationship between demand and price is the same for both people, whoever is more eager perceives a lower "price" to pay for forming the relationship. (And so the less eager person perceives a higher "price.") Again looking at what you've seen in your own experience, or judging from cultural depictions, we know that it's the geek who's more eager to begin the study group -- by a longshot. He gives up an hour of his time, which he probably wouldn't be doing much else with -- maybe some resume-padding stuff for college applications -- and he gets what amounts to a low-pressure date with a cute girl. She has to give up an hour of her time, which is a lot more valuable -- she has a lot more that she could be doing, being young, pretty, and popular -- and she gets a small increase in her grade.
Since he jumps at the chance, while she goes along grudgingly, if anything the geek is the parasite and the cheerleader is the victim. Again, both profit from the exchange, but the closest parallel to the host-or-parasite question shows that we should feel sorry more for the cheerleader. Most smart people would automatically see the geek as the parasitized -- she's just using him to get a good grade, like all those other times he's had to pull all the weight in group work. But if you step back and look at who's more eager than the other, you see just the opposite -- that poor doe-eyed cheerleader has to suffer the creepy presence of such a geek just for a measly boost to her grade.
Arts people are generally smart but not good-looking, so most of the images we're familiar with here may show both sides, but they're much more favorable to the nerd. They reveal his not-so-pure motives, but they're shown as far less despicable than the cute girl's shallowness and boredom while around him. How different our picture of the world would look if cheerleaders wrote the social history books.