I dont get it myself. I have had rabbits for more years than I care to count and of many different breeds through all the years and I never had an outbreak. In fact diarrhea is very rare here.
I also never feed fruit or veggy as buns are herbivores. I never feed anything from the grocery stores. Pesticides and herbicides are all in the produce. My buns dont need that pioson.
Only herbs such as wild weeds, shrubs, branches, and grasses will I feed and pellets for balanced nutrition and hay free choice of course.
Hay is very, very important for good digestion and to keep teeth worn and wool block away.
Type of hay doesnt matter as its not so much a food but a fiber source. Stemmy grasses are whats needed.
IMO, its often a management issue.
To much bad food and not enough good food and hay.
Oh and exercise!!! I believe animals like people need exercise to maintain health. I dont hear much about people taking care of exercise needs.
I have over 20 buns right now. Cages are 36x30 and I rotate buns in a fenced pen I put out so they can play.
I try to make sure each bun gets out for play at least once a week.
I hope to build a few more pens so I can let out several at a time, one to a pen.
In winter they just get turns running loose in the rabbitry.
Also breeding. Breed the best and cull the rest. Thats another important issue needed to prevent the breeding of weakened or defective animals.