Viper, it is true that an individual's experience can be incomplete or just plain wrong. To counteract this, most of us base what we consider true on more than one person's experience. This still doesn't guarantee truth, of course.
In the example about disease you gave, at least one person would have to test the person's theory (and ask him/her to explain the methods used to arrive at the knowledge).
Margaret suggested Ontology. I would also suggest Epistemology: the study of how we know what we (supposedly) know. Sometimes, when experience is not possible, all we can do is assume it to be true, and then hopefully try to verify the assumption.
