Behavior-altering parasite

A behavior-altering parasite is a very small creature that lives inside other animals (such as birds or insects) and affects their behavior. It does this by sending chemical signals to the host's brain, changing the way that it behaves. For example, some parasites can make their hosts less shy, or even more likely to be eaten by larger animals. This can affect the entire population of the creatures that the parasite lives in, since their behavior could change the way they interact with the environment or other animals.