Nanomachine Rejection is a medical condition where the body reacts violently to the nanomachines infesting it. Since everyone in Glitch City (and possibly the world) has been infected with nanomachines in order to keep track of them, a small number of people either are suffering from or know someone who is suffering from Nanomachine Rejection.
Nanomachine Rejection is almost always fatal, with only a small percent of people surviving the onset of NMR post birth. If an unborn child is diagnosed with NMR, it is possible to turn them into a Cat Boomer while they're still in the womb.
In the year the game takes place, there have been 80 cases of NMR reported in Glitch City.