When Gap Drives were made, and humans started moving out into space due to it’s invention, a mysterious and potentially deadly flaw in the human mind was discovered. For a very small percentage of space travelers, crossing the gap will trigger some undetectable flaw in their mind that causes a form of psychosis or derangement when certain conditions are met, or in very rare cases, this may take effect permanently after the gap crossing. These individuals, if they can be properly restrained, and if the situation is right, they can be fitted with an NMI.
Examples of gap sickness includes a sudden psychotic outburst due to external stimuli, such as space debris in close proximity, going EVA, or from heavy G from going full burn. Some more severe cases are permanent in their effects, and can put a crew in a very dangerous situation after their first gap crossing. There have been cases of gap sickness where victims of this bizarre condition will go on a murderous rampage, or sabotage their ship in some way.
There have been many theories and almost as many studies done to try and determine what makes someone suceptible to gap sickness, or how to cure it, but so far it’s all speculation. It seems to be able to affect anyone in any way, but if you don’t succumb to gap sickness after your first crossing, you will not succumb to it on any crossing made after that – this much has been observed.