Tom's logic is, of course, actually correct.
His argument is that if your immune system is strong enough (strong being far more complex than just energy or white blood cell count) then it can defend against any disease.
HOWEVER
Human immune systems are not and can not be strong enough. The Human immune system works on various principals of biology, protein chains, and a lot of stuff I don't understand. It is finite in it's ability to produce antibodies and it can only produce antibodies for things it recognizes as a foreign containment.
Here's a good article on how pathogens avoid being destroyed.
https://www.ncbi.nlm.nih.gov/books/NBK27176/Now, if you have an immune system that's somehow evolved to not be susceptible to any of these tricks, you're immune to disease. But human immune systems are not like that. A few are immune to AIDs due to a missing protein that HIV needs to attach and infect a t-cell.
http://www.hivplusmag.com/research-breakthroughs/2016/3/23/anyone-immune-hivThis isn't a strong immune system, just a different one.
Also, Rabies apparently has about a 100% mortality rate. Plague too.
Also, isn't Ebola at 85-90%? Where's the 50% figure?
Anyway, diseases usually kill when whatever they do is done faster than the body can remove them. And so far, the addition of medical science has not shown any increase in mortality rates. Quite the opposite, really. Before modern medicine, death by disease was far more likely. The human immune system can only do so much.