blood-brain barrier

The brain has evolved a method of protecting itself called the blood-brain barrier. But the BBB can backfire when it prevents drugs to treat brain diseases – for example, a malignant brain tumor – from reaching the site of the disease. Science has come up with some clever methods for defeating the brain's own defenses.
Regardless of our brain's natural security, some pathogens still get in. One is called Cryptococcus neoformans, a fungus that can be found in the excrement of flying tree rats, like pigeons and bats. After being inhaled into the lungs, the fungus makes its way into the brain using a clever mechanism.