Your creatures are immune to antigen 7 and its sub-effects.
I just did testing on some fresh generation 1 (2017) ChiChi Norns in the Grendel Jungle, using an altered bacteria script that only infects Creatures with antigen 7. They are being infected with antigen 7 and are being injured so far.
Were you by any chance seeing this in Ettins? They are always unaffected by antigen 7.