Hidden Epidemic: Tapeworms Living Inside People's Brains

From Discover Magazine:

But sometimes tapeworms take a wrong turn. Instead of going into a pig, the eggs end up in a human. This can occur if someone shedding tapeworm eggs contaminates food that other people then eat. When the egg hatches, the confused larva does not develop into an adult in the human’s intestines. Instead, it acts as it would inside a pig. It burrows into the person’s bloodstream and gets swept through the body. Often those parasites end up in the brain, where they form cysts.

The tapeworm larvae often get stuck in ventricles, or fluid-filled cavities, in the brain, sprouting grapelike extensions. In this way the worm actively cloaks itself from immune cells. Protected and well fed, its cysts can thrive there for years.