It's the one where they immune system of some genetically engineered children was designed to seek out pathogens outside their bodies so it could destroy them before they even had a chance to attack them. Their immune systems end up attacking any non-genetically engineered humans they encounter. That unintended side-effect is where these researchers got their idea from.

It's also ironic that on what was a progressive show at the time, this episode was written as a cautionary tail about genetic engineering and eugenics. These day the progressive message is close you eyes and blindly trust the experimental science as it mucks about with your cell's machinery. What could go wrong?

The pre ultra-SJW Star Trek had lots of episodes about the unintended side-effects of technology and mucking around with the human genome. Something that wouldn't fly today with things such as these experimental vaccines, the metaverse whatever that will turn out to be, and the social credit score they're going to start pushing very soon.