How do infections cross the blood-brain barrier?

Depends on infection. Some bugs( viruses, bacterias, etc.) are small enough , and if they circulate in blood, can easy cross the barrier, others require trauma or surgery to penetrate the brain.

Related Questions

Do fungal infections cross the blood-brain barrier?

They can. While the blood-brain barrier is a critical regulator of interaction between the blood stream and the brain, it cannot keep everything "out". Many infections, including fungal can spread to involve the brain. Some very serious fungal infections commonly involve the brain (central nervous system-cns). Read more...