Fix for #5532 and the last of the infection changes, I hope

Fixes the internal organ processing issue spotted by CIB and adjusts a few other things I realized that needed to be looked at.

* Fixes #5532 
* Increases the time required for organs to become infected from wounds to five minutes on average
* Removed the now unnecessary get_cure_threshold() proc
* Increased the severity of infection required for internal organs to take damage
* Some infected internal organs have visible symptoms. E.g. Infected lungs make you cough.
* Germs now spread from external organs to internal ones one at a time, instead of every organ at once.
* Minor wounds are now much harder to infect
Right.