I was watching Everything, Everything this morning and the illness mentioned in the movie made me curious on whether it was a real deal, and not just a made-up, imaginative kind. So I searched through the internet as I was doubtful that such kind of illness really exists. And turned out that it was... or is a real disease.
SCID is a group of very rare, life-threatening diseases that are present at birth. The disease causes the child to have very little or no immune system.