Parasites as Medicine

The most striking evidence comes from multiple sclerosis, a disease where the immune system attacks the brain and spinal cord.

In one experiment, MS patients who happened to pick up intestinal worms developed fewer new brain lesions on their MRI scans. When the worms were later cleared out, the lesions came back. The body seemed calmer with the parasites than without them.

Why would worms quiet a disease like that? They survive by actively damping the immune response that would otherwise expel them — and we now know much of how.

Worms ramp up the body's regulatory T cells, the peacekeeping cells that hold inflammation in check, and trigger calming signals like the molecule IL-10. Those same brakes can rein in an immune system that has turned on its owner, which is exactly what the MS scans suggested.