Baylisascaris procyonis is a nematode parasite, related to Toxocara canis. The adults live in the intestine of their hosts and lay eggs that pass out with the feces. If an animal swallows these eggs, the larvae hatch out in the intestine of this host and then one of two things can happen. If they find themselves in their final, i.e. definitive, host, then they will basically stay put - maturing into adults and beginning the cycle all over again. But, if the animal that swallowed them is not the definitive host, then well, things get a little messy. In these cases, the larvae leave the intestine and travel through the bloodstream, invading other tissues and usually making temporary homes in the central nervous system. If that host is a mouse, they can kill it or at the least cause it to act strangely, which can increase the chances of it being consumed by the definitive host. B. procyonis is a parasite of raccoons, and is very common in them (prevalence >70%). Recently, with raccoon populations expanding and coming into closer and closer contact with humans, people, especially children, are increasingly becoming accidentally infected with this parasite. Since we're not the definitive hosts, the larvae undergo visceral migrans and can cause very serious disease and even kill the human. There have been at least four reported deaths due to this parasite in the U.S. since 1980 and there is not currently an effective treatment.
You can read more about the human cases of B. procyonis here.