A Taboo Child (忌み子) is an abnormal human that inherently attracts curse towards him. Humans are born with this condition and, because curse naturally gravitates towards them, large concentrations of curse are accumulated near them. It is a very rare occurrence, but every now and then they're always found in every era. Due to their misfortunate circumstances that lead to gather curse easily, aberrations take place very frequently around them, even more so if there is no God of the Land to purify it or if he's incapable of doing so for whatever reason. Since the creation of amasogi is common around them, people that know of their nature tend to avoid them or even lock them up in isolated locations.
The conditions for a person to be born as one are unknown, but a pregnant woman getting exposed to a large concentration of curse could potentially be a major reason cause. The only taboo child known is Kazuya Kagami, though it is very likely that there are more.