Parasites passed on via the food chain often influence the behaviour of their host to their own benefit. One example of this is the tapeworm Schistocephalus solidus, which makes three-spined sticklebacks behave carelessly. The infected fish venture more often into open waters, making themselves easier prey for piscivorous birds, e.g. kingfishers. This is just what the tapeworm wants, because it reproduces in the bird’s intestines. A team of evolutionary biologists around Dr. Jörn Peter Scharsack at the University of Münster (Germany) have now demonstrated for the first time that the tapeworm not only influences the behaviour of the infected fish—indirectly, it can also induce equally risky behaviour in other healthy fish in the group. The study is published in the current issue of the journal Proceedings of the Royal Society B.