This was something I watched on UK TV in the late 1960s, (maybe a series like The Outer Limits or The Twilight Zone).
A large team of Earth space explorers are visiting some planet, lots of ground mist and the aliens live in like polythene and tinfoil wigwams!
Some of the natives, who are humanoid but with big heads IIRC are ill and act all drooling and uncoordinated, the Earth doctor examines and identifies a germ in the aliens blood.
He rapidly develops a cure and his team spray it throughout the alien town.
Soon all the aliens are drooling and uncoordinated which is when realisation dawns....that is the natural state of the host bodies; it was the germs that were the intelligent part of a symbiosis.
Edit:- After looking at comments and suggestions it's possible that I watched this and later read the Star Surgeon story and got the plots confused....germs that eat metal and germs that convey intelligence.
Thanks all