The statistic (which is not recent, I agree but I think the factsheet gives a good breakdown of the causes of homelessness) is that of those who are chronically homeless 80% substance abusers and 60% mentally ill. Obviously there is overlap, so amongst the drug users are mentally ill people. So we have co-morbidity of conditions - and according to the DSM substance addiction/dependance is itself a form of mental illness. Once a person is addicted to a substance, whether you think it's an illness or not, they are no longer operating with a properly functioning brain. Beyond that, even if a drug dependent person wants to stop using, it requires the help of professionals and a support system in order for them to do so. Labeling people with mental illness, substance abuse problems or who are poor as : Lazy, bad, weak, or choosing their situation is just a good way for people and society to avoid helping people who have been disadvantaged by birth or circumstance.