Sanitation issues and lack of vaccination.
Typhoid Fever, for example, is an oral-fecal illness. So, sanitation is huge.
Plague is due to an overpopulation of rats. This is transferred through their fleas.
Measles is due to lack of vaccination.
Hepatitis is related to needle exposure (getting poked by needles that are left in bathrooms, on the streets, etc.) You can get vaccinated for this. Healthcare practitioners are required to get this vaccine. But, if you are exposed to someone's blood that may be infected, go to an urgent care center, or the emergency room immediately.
Same if you ever get bitten by an animal, such as a rodent, cat, dog, or other animal if you notice any foaming at the mouth, etc. (Particularly a possum or a raccoon.) Rabies is 100% deadly if you don't get the vaccine immediately! They use a dead version of Rabies.
I hope this helps answer your question! :)