How can your parasites become your allies?

Although parasitic infection is usually detrimental, it can be beneficial to the host in some situations. Parasites could help their host by providing a new function or modifying one of the host's life-history traits. We argue that the evolution towards a lasting mutualistic relationship would be more likely when parasites endow hosts with new abilities rather than alter a trait because hosts are less likely to evolve a new capability on their own than adjust their life history by microevolutionary steps. Furthermore, we underline how evolved dependence – the host's loss of ability to live alone owing to a long history of evolution in the presence of its parasites – has shaped contemporary mutualistic relationships.