Most people agree that bacteria can cause disease.
Salmonella, meningitis, gonorrhea, are all bacterial diseases. The cause of disease symptoms and the understanding of the idea of disease used to seem very straightforward. Bacteria got into a persons body and you got sick. The invaders swarm in and cause all kinds of mischief until the body rallies and there is a dramatic counteroffensive by the immune system and the invaders are destroyed. It seems like a simple model with the good guys and the bad guys, just like in the movies.