A T-cell stimulator was a medical device that can be used to stabilize a patient's immune system by stimulating T-cells, as the name implied. In 2367, Doctor Beverly Crusher used a T-cell stimulator to fight Susanna Leijten's infection with the Tarchannen III species. She hoped the device would slow down the rate of change and eventually inhibit the transformation. It turned out the infection was parasitic in nature and that the parasite was hiding in Leijten's thymus, producing the changes similar to a viral crèche making the T-cell stimulator ineffective. (TNG: "Identity Crisis" )