|An infected grup|
Grups, a contraction of "grown ups", was the word used by the Onlies, or children, of Earth Two to refer to adults. On that world, adults and children entering puberty developed the fatal form of the life prolongation project. Blue blemishes covered their skin, and they became insane and dangerous. In this state, the adults attacked anyone near them, and destroyed their surroundings.
During the initial outbreak of the plague, three centuries earlier, the children were forced to hide from all adults, including their own parents, until the adults died. This left the children with a deep-rooted fear of and distrust for adults. (TOS: "Miri")