The body's organs are fed by blood, which provides nutrients in exchange for waste products from cells. This free exchange of goods and services, however, is dangerous for the brain, primarily for two reasons.

First, the brain is under tight metabolic control. If ions and other substances had easy access to the brain, neurons would not work properly. Second, if there wasn't an extra layer of protection, nasty microbes could gain access. To prevent both of these unpleasant outcomes, the brain is protected by a special wall, called the blood-brain barrier (BBB), that tightly regulates what goes in and out of the brain. 

Despite this extra security, some pathogens still get in. One of them is called Cryptococcus neoformans, a fungus that can be found in the...