Instead of having a flat infection growth rate simply, I propose all injury inflicted by zombies infects the player to different degrees. This would be based on the type of injury, and how many the player has stuffered before overcoming the Knox infection.

I would base it off of the vanilla zombification chance rates, stratches giving the smallest amount, lacerations a 4th of a bite's worth, with the bite being the maximum infection would be applied in a single injury.

If the player has 4 lacerations from zombies, then they would have a bite's worth of the Knox Infection. This would mean if you got mauled, it would take a lot more nursing to recover than minor injuries.

The idea is you could shrug off minor injuries, but too many and your immune system would be completely overrun even with every possible boost.