Final answer:
Microbes can cross the blood-brain barrier through intercellular entry, transcellular entry, and nonhematogenous entry.
Step-by-step explanation:
Microbes can cross the blood-brain barrier in several ways. One way is through intercellular entry where they use virulence factors, toxins, or inflammation-mediated processes to pass between the cells of the barrier. Another way is through transcellular entry where they pass through the cells of the barrier using virulence factors that allow them to adhere to and trigger uptake. Additionally, microbes can enter the brain without encountering the blood-brain barrier through nonhematogenous entry by traveling along the olfactory or trigeminal cranial nerves.