With that many people, an outbreak of cholera or typhoid fever would not be unheard of. Go home, we don’t want your germs!
“we don’t want your germs!”
Or cooties for that matter.
With so many of them being infested by lice under highly unsanitary conditions, there's also an excellent chance of typhus breaking out. Typhus is what killed over half of the army Napoleon took to Russia in 1812. See:
Stephan Talty, The Illustrious Dead: The Terrifying Story of How Typhus Killed Napoleon's Greatest Army