Subject: why do we get infections , and why peopel die from it

We know what kinds of organisms and entities
cause disease, how they do so, and how to treat diseases
as well as prevent them through such things as
vaccination and good sanitation and personal hygiene. 
Given what we know, why is it that people (many people)
still become ill and die from infectious diseases?  

We covered this material in class but I,m stil not clear why do  we still get 
sick

Re: why do we get infections , and why peopel die from it