To be clear, there are many types of homeless in the US. But the ones that get the most attention and seem to cause the most problems are those pushed into mental illness by drug use. And, yes, drug use does not always cause mental illness and many mentally ill are not on drugs, but as those of us with direct experience with meth and other 'hard' drug users can attest to, a meth habit is not conducive to mental wellness. There is an acute phase of reality detachment that gets better with cessation, but even then, the hard core meth addicts I've known have mental scars that linger indefinitely.