On the topic of diseases

First of, i would like to ask what exactly affects the speed at which diseases and immunity progress? I know that treatment quality, blood filtration, bed rest, being well fed, hospital beds and vitals monitors affect these things. Things i am unsure about are colonist age, blood pumping and does the use of better medicine have an effect beyond affecting treatment quality. Furthermore i am unclear if these things affect only immunity buildup or do they slow down the disease progression as well. I once had a colonist with malaria and with good medical care the progression of the disease reversed i.e. started running from 70% to 69% and so on. Eventually the disease progression fell below the limit whereby the disease is hidden and the disease vanished. It of course appeared again after the latest treatment wore of and the progression again climbed over the threshold. I wonder if this was a bug or can good enough medical care actually reverse the course of a disease?