Parasites do not just cause disease. Sometimes they go further, actually manipulating the behaviour of the host. For instance, mice with malaria are more likely than other mice to be out at dusk ­ prime feeding time for mosquitoes. Nor is manipulation restricted to behaviour. The insect parasite Wolbachia sometimes causes its insect hosts to reproduce asexually ­ an outcome that benefits itself more than it benefits the host. Some other insect parasites are what are known as ‘male killers’. These parasites, like Wolbachia, are transmitted vertically through the female’s eggs. But if they find themselves in an egg that is going to be male, they kill it. This way, their relatives in other eggs will have a better chance of survival.
