Dude gets an ingrown hair. It gets infected, now he has a boil. Ought to go to the doc and get it lanced, but he doesn’t. Weeks pass. The boil grows, eroding downward toward his skull. Ought to go to the hospital in a jiffy, but he doesn’t. Weeks pass. The ingrown hair infection reaches his skull.
Bone, once infected, presents little barrier to spread of infection to contiguous bone, and so it spreads within his skull. Ought to spend a good long time in the hospital, but he doesn’t. The bone dies, and begins to erode. Weeks pass. At some point, the smell attracts flies, which begin to lay eggs in his festering wound, and maggots take hold. Weeks pass. The infection breaches the inner layer of his skull, and reaches the meninges. Weeks pass. Though their tensile strength is impressive, the meninges are quite thin, and the infection breaches them.
Now, infection and maggots set to work on his brain. Your brain just isn’t supposed to be on your outside, and presents almost no barrier to anything when exposed. Infection and maggots get to work on his brain. This makes him feel a little wobbly on his feet, and so, what do you know, he decides to see the doctor.