The Parasite in the Brain

Toxoplasma gondii is a single-celled parasite whose final host is the cat. It can infect many warm-blooded animals, including humans, and can form long-lived tissue cysts.

In rodents, infection has been linked to reduced fear of cat odor and broader changes in risk behavior. In humans, behavioral effects are much harder to prove, so the safest wording is influence under study, not a parasite steering personality.

Almost everyone who carries Toxoplasma feels nothing at all — and that silence is the whole trick. A parasite that made its host obviously sick would get noticed, treated, and wiped out. This one wins by staying invisible, settling in for the long haul while it quietly nudges its host from the background.