I was reading an article recently about how homeless have stress levels that are so high that their hippocampus (a physical portion of the brain) literally shrinks, causing effects such as short-term memory loss, traumatic flashbacks, severe mood swings, depression, anxiety, and irrational behavior. Basically, they're experiencing PTSD, without the "post-" part.

When every minute of your day is spent worrying about where you'll sleep that night, if you'll be safe or mugged or raped, when you might eat again, and whether the police are going to try to put you in a cage, the synaptic firing and responses in the brain re-wire themselves to act as if you are in a constant state of fight-or-flight--and you are. This causes physical changes to the brain and makes it even harder to crawl out of that hole.

It can be reversed though, with therapy and drugs that stimulate growth in the hippocampus; but of course for those to work, the person has to be physically in a place where they are safe and have the basic necessities taken care of. Without those things, it's just a vicious downward spiral.