Ah, well if you're poor & living in a poor neighborhood then the following are more likely:

  - Low income tends to imply low education, so maybe not
   fully understanding the risks of HIV. 
  In some parts of Africa they think(thought) 
  that sleeping with a virgin would cure men of HIV. 
  That's an education problem in the poorer parts of the continent.

  - Prostitution is probably nearby.

  - Stress of life living paycheck-to-paycheck, can't 
  afford vacations, one of the few things you can enjoy 
  at some random parties & nightclubs is random hook-ups
  with whatever person you can get.

  - Drugs an unclean needles 'n such.

With well off people those above still happens, but in a different way sorta.

- middle-class to wealthy understand how HIV works for the most part and will more often avoid the risky behaviors

- Prostitution is not needed when you can hook-up with the random co-worker or at some university party with smart people who are more likely than not HIV-free. Then there's what I like to call "high class legal prostitution": http://www.prlog.org/12074839-gold-digger-cruise-connects-be... We know what's probably happening on that cruise... Also expensive high class gentlemen's clubs...

Drugs for middle-class/rich? They don't have to buy/share them off the street from random people like poor people have to. They order from reddit's sheepmarketplace or silkroad!!!