According to this book, what makes a parasite or a disease successful is its ability to perpetuate itself.
While parasites and diseases often make us sick, that is not their purpose. There may at times be ways in which they benefit by making us sick. For example, if they can be spread by body fluids and they give us colds, they can be spread by the fluids that are ejected when we sneeze or cough. But it is not necessarily good for a parasite to make us very ill. In particular, it can be bad for them if they kill us because that may well end up killing them as well.
Therefore, a successful parasite or disease is one that does not cause its own demise. Instead, a successful parasite or disease can live in an individual or a population for an indefinite amount of time. Success has to do with longevity, not with making people sick.