In mine, septic is pretty prevalent.

Income: Scheduled rents/leases and other income (rubs, NNN, parking, laundry, all less prevalent in comm.)3.

This being said, there are countries in which corruption is way too prevalent but this is the thing.

Knob and Tube wiring is prevalent in Worcester in a couple of the neighborhoods, but not so much in others. 

It isnt something Id give any consideration to due to the prevalence of them. 

You'll see this more prevalent in outlying areas or rural areas.

For me, I felt noticeably different for several months and have no side affects today after extremes short term exposure.Needless to say, I'm a big advocate of CO detectors being prevalent in the house and being wired in with battery back up.

The prevalence of plexes seems to vary a lot by market, and Charlotte is just one of those markets with very few of them. 

And does it seem more prevalent in lower class housing?

Again, a property that is 40 years old but in similar area with similar few amenities and updated would still fall into C class and not drop to D on age issue alone.D class is typically over 40 years old, in bad neighborhoods, crime is prevalent, no amenities, and tenants are typically at or just above poverty levels (most on government assisted housing.