Walking around in horseshit and -piss all day before most medical science had been discovered can't have been great for public health.

It's good to remember you can't see the smell in old photos!

Wow, good thing replacing all that horse shit and piss with leaded gasoline for half a century didn't have any negative effects.

And now cars put far worse things than horse shit right into the air.

Horse poo is mostly chewed hay, not dangerous.

It can contain listeria, giardia, roundworm, e. coli, salmonella, clostridium and all sorts of other nasty business. Most healthy horses have low amounts of these in their stools, especially compared with dogs or humans. However, each horse produces about 5 tons (11,000 lbs) of poo a year.

A bunch of horses in a big city on a rainy day is a recipe for spreading disease.