Starfield:Decontamination

Decontamination improves the rate that you recover from infections on your own. The highest rank of the skill also provides a 20% chance that you will not gain an infection at all when you normally would. Decontamination is unlocked by spending a total of 8 skill points in the Physical skill tree.

Skill Increases
The fastest way to quickly progress through the skill rank challenges would be to find a place where you were certain to gain infections, while having numerous items to cure them. Continuously expose yourself to the adverse conditions to gain the infection, while curing it just as quickly.