When it rained, all that waste turned into a slippery mud, and when the weather was dry and windy, people were coughing up lungfuls of poo-dust. (Think about that the next time you're whining about pollen.) And as you'd imagine, diseases were rampant. Big cities were basically a Fallout game that replaced a nuclear bomb with so, so much horse feces.
And then there were the pigs. Hog farming was big business, and there were so many that farmers gave up keeping them on farms and simply let them roam the streets in great packs (there was plenty of garbage around for them to eat). Unfortunately, the dogs and pigs would then also defecate in the streets, creating a wonderful cocktail of excrement. When Charles Dickens first visited New York in 1842, he snarked hard on Broadway's hog problem, writing that the pigs lived a "roving, gentlemanly, vagabond kind of life" on "equal, if not superior footing" with humans. That's a clever way of saying Get me the hell out of here.