ROUNDWORM LARVA.

It bore tunnels through the brain tissue.  It causes headaches, poor memory, losss of concentration and depression.

TAPEWORM CYSTS.

Structurally damage brain tissue.  They absorb nutrients and thus steal from the brain's nourishment.  They excrete waste into the tissue.

GRANULOMAS.

Granulomas are large cysts.  They may contain living or dead parasite cysts, living or dead eggs or broken cell walls and other cellular debris.

PROTOZOA.

Most species of protozoa live in the small intestine.  Most also invade other tissues including he brain.

SAPROPHYTIC PLANTS.

Saprophytic plant parasites invade all tissues including the brain.  These are fungus, slime mold and mildew.

MICROZYMAS, MYCOPLASMAS AND NANOBACTERIA.

These can also invade and cause damage to the nervous system.