I think there are a number of "automatic" or instinctive reactions to things whose advantage is that they do not require thinking; thinking can be quite difficult, and time-consuming. Isolating or shunning those with illnesses (or differences) may be an automatic evolutionary way of reducing their chances of procreating and maybe propagating the illness (or difference, which is seen as threatening).

Human history (or pre-history), I think, shows many examples of these automatic responses to anything different, and it is often quite difficult to sidestep whatever is automatic to look more closely into what is causing the illness or discomfort. The earlier animals had to depend on instinct even more than we do, as they had less capacity to think things through than we do. Obviously we still have a lot of difficulty setting aside our fears.

Thinking "outside the box" takes work.