Humans aren't the only ones that can benefit from parasites; in the animal kingdom there are several examples of two different organisms living in close association with one another. In cases where one organism leaches resources from its host, it's usually defined as a parasite, but inter-species relationships are rarely quite so black and white. Symbiosis is when organisms of different species are able to work together for mutual benefit, transcending the parasite-host relationship.
One of the more famous examples of symbiosis is the red-billed oxpecker, a medium-sized bird found in sub-Saharan Africa. The oxpecker rides on the back of large mammals, like rhinos or water buffalos, feeding on the ticks that are found on their host's back. So, in this peculiar relationship, the oxpecker gets a free ride and a meal, while relieving the host from an unwanted blood-sucking parasite. Everybody wins, right?
Typically, the oxpecker-mammal relationship has been thought of as symbiotic or mutualistic, but a recent study published in the journal Behavioral Ecology showed that while the birds do indeed benefit from their relationship, they don't significantly decrease the number of ticks on their host. And to make matters worse, from pecking at the ticks, the oxpeckers can slow down the healing time of wounds, and they can also remove earwax. Oxpeckers feed on blood (that's of course why they're so interested in ticks), and they have been observed sucking the blood of their hosts and even opening new wounds. Suddenly this relationship is starting to sound a little more one-sided [source: Weeks].
There's one more major factor that should be taken into account, though: When oxpeckers rest on the backs of large mammals, they can serve as an early warning if predators are approaching. So are they parasites after all? Let's just say that oxpeckers have a complicated relationship with their hosts [source: Nature].