This is a cyst inside the brain of a mouse
It is filled with the parasite Toxoplasma gondii
This parasite spends one stage of its life cycle in cats, and the other in different mammals - including humans. The parasite causes flu like symptoms in the host, and can rarely be fatal. When the parasite is in the brain of a mouse, it will change the animal’s behavior so that instead of being afraid of the scent of a cat, it is drawn to it. By doing this, the parasite gets a quick ride back into the stomach of a cat so it can reproduce and start over.