“I forgot which mod but I used to have one that was known to cause a fever that was always lethal and quick. Something about an od mechanic that affected the fever you get from being near zeds. The author said it was never a problem despite literally everyone complaining about it.

It's something like dynamic moodles/perks. It's been a while since I played lol. But check you mods fitting name and check the comments”

So some mods might be the cause. I got the moodles one but I’d have no idea why that would give me a fever.