I generally agree with your article, but it has at least one false premise:

Something discrete; a graph would show two widely separate populations, one with the disease and one without, and not a normal distribution.

But many undesirable conditions that are caused by genetic or environmental sources are continuous. Cancer is actually one of them, as far as I understand: there are many different kinds of cancer, and the symptoms can vary in severity (though all are fatal if left untreated). The common cold is another example, though of course it is rarely fatal.