100 years ago, the average american had no idea how common slums were outside the US.

This was and still is very true. The level of the poverty in places like that is astounding and beyond the experience of most anyone in a 1st world country. I grew up in America, in poverty of the level that my single mother was only eating what she could scrounge at work some years so she'd have enough to feed us kids. Yet when I deployed to Panama in the mid 90's for a 2 month military operation, and had to operate in many of the rural areas of Panama during those missions, I had my eyes opened to what real 3rd world poverty looks like. The way I grew up would have been a huge improvement for many of the people I saw there. You can't really understand it until you've seen it with your own eyes.