You Are Probably a Host

Parasites are not a tropical-travel problem. Right now, roughly half the world's people carry at least one parasitic species, and most have no idea.

The clearest example is on your own face. Demodex mites, tiny eight-legged relatives of spiders, live head-down inside the hair follicles of nearly every adult, quietly feeding on the oil your skin makes. You will never feel them — and you almost certainly host hundreds.

Demodex are not alone. Pinworms are the most common worm in the developed world, especially in children and their families. Toxoplasma gondii, a single-celled parasite, has quietly settled into roughly a third of all human beings.

Here is the strange part: almost none of these hosts ever feel sick. These are not invaders breaking in for a quick attack. They are permanent roommates you never agreed to house, and your body mostly lets them stay.