I think that the likeyhood of Getting the condition may be determined by your state of health. OTOH, sometimes tiy can just be damned unlucky! Once you get it, your immune system may fight it off better than other, you might not. Again, some of this is down to your general heath, but some of it is just luck.
With dome inllnesses , it doesn't matter how good your immune system is, if you develop the disease , you are basically fucked. Rabies being the classic case . One you get rabies, ie if your immune system doesn't do a good enough job, and you develop the condition you are dead.
The Doctors comments were that bacteri etc do not cause diseases. This is total and utter bollocks.
While you can improve your chances of surviving by being, say not malnourised, obese, very old and very young, if you don't get the pathogen, you don't get the illness.
So I am polar opposite to the doctor quoted at the start.
HIV is one of the most undetstood of all viruses BTW, I can google for lots of pictures if you wish