It's a fungus that crates these massive toxins. Kind of a fail save for those that where never supposed to survive this. If disturbed by removing SLH and other critters as well as radiation it start to work..
This has been going on for about a week but the dramatic increase in SLH removal and radiation removal have hit you hard.
The timing used now will hopefully stop this, via dimensional work we can get rid of the fungus, its enzymes and toxins before removing the residual isotopes..