There ARE "good" diseases, we just don't call them that.

Our body is filled with micro-organisms that help us digest our food, help stimulate our immune systems, and perhaps even help us fight off invaders. I wouldn't doubt they do a lot more, too. I know doctors warn us that keeping overly clean and excessively antibacterial can make us vulnerable to infection. And in some cases, some diseases, like sickle cell anemia, give some protection to malaria, although that is a mutation, not actuyally a disease.

I bet we are commensal with many bugs, although we don't feel the need to study them as intensely so we know little about them.

"We are all a multitude."