The main thing connecting them is the economic level of the areas that lack sanitation. Another characteristic of these areas (particularly in China) is that humans and domestic animals (chickens, pigs, goats, etc.) share the domicile. This close proximity of humans and animals has been pointed out as one of the main reasons that new flu strains tend to originate in Asia. So while the lack of sewage treatment is a health problem, it's probably not the cause of new diseases, but the conditions that foster new diseases are endemic and related to the overall "health" situation.