From MSNBC:
Parasites are by definition bad for you. Some, such as malaria, can kill. Others, like microbes known as Wolbachia that are found in more than one-fifth of all insects, often make female hosts less fertile. Now scientists discover parasites can evolve surprisingly rapidly to become helpful instead of harmful. The typically nasty Wolbachia can make females more fertile instead of less, a study reveals. They’re not doing it out of the goodness of their non-existent hearts — they boost host fertility to better spread themselves in nature. For instance, Wolbachia parasitizes a worm that in turn parasitizes humans, and this worm already depends on Wolbachia in order to produce young.
Wolbachia are bacteria that insects get only from their mothers. They can display a bewildering diversity of additional effects, such as turning males to females, causing infected females to reproduce without males and triggering vicious cycles of increasing female promiscuity and male sexual exhaustion. The presence of these parasites also often carries a toll on their victims — for instance, cutting down the number of eggs that females produce.
More here.