The Darwin Genetic Research Station on Gagarin IV engineered children whose immune systems caused a deadly ageing disease.
The Darwin Genetic Research Station is a Federation facility on the planet Gagarin IV, led by Dr. Sara Kingsley. In Unnatural Selection the Enterprise traced the death of the USS Lantree's crew to the station, the ship's last port of call, and found its staff ageing rapidly.
The station had been creating genetically engineered children with immune systems that actively attacked disease. Those defences were causing the ageing in others. After the crew used the transporter to cure Dr. Pulaski, the children were kept in isolation, and the Enterprise destroyed the Lantree to prevent further spread.