This discussion is on a randonneuring list I'm on. The first guy is talking about some race in France and why fenders are a good idea. Apparently there had been a lot of rain, and a lot of cow traffic on the road.

"When the riders sped through these stretches, the excrement
sprayed out in all directions -- onto the riders' faces and onto the
mouthpieces of their water bottles. Consequently, when a rider
took a swig from his bottle, he also unwittingly swallowed millions
of E-coli bacteria. Within 12hours of the E-coli contamination, the
riders suffered severe upset stomachs with vomiting and diarrhea."

Second guy replies:

Several years ago on a brevet in southern Wisconsin, we experienced the same type of weather and manure covered roads. Interestingly, those who were using camel bag-type hydration packs did not have same problem of those who relied on water bottles.