Thymus
From Memory Alpha, the free Star Trek reference.
The thymus is a gland located in the upper chest of a Human body. It plays an important role in the Human immune system.
The parasitic Tarchannen III species cells were located in the thymus after infecting a host, using the immune system to spread genetic instructions that would turn the host into a full member of the species.
Doctor Beverly Crusher successfully removed the parasite from Susanna Leijten's and Geordi La Forge's thymuses after they were infected by the lifeform. (TNG: "Identity Crisis")
