Edit: You RAW Nazis out there may argue the line 'any particular enemy unit' does not specifically say that a unit in your army did not cause the wounds, it just points out that it wasn't a specific one. But this is an insane reading at best and only leads to some very weird mind warping around this line.
HA! I knew I would catch one.
Anyway to wrap your head around this logistical nightmare, you would have to have a full army of only nurgle units. Because if you choose to argue that it wasn't any specific unit in the army that did it, your opponent can simply point out that unit of Deamonettes and say well I think they did it, or perhaps that soulgrinder in that corner did it, or maybe it was that grot somebody left on the table, or perhaps you should stop being such a
RAW nazi (not you, just pointing out a possible conversation to spring forth if you pursue that line of thinking.)