Many a mangy mongrel loose and roaming the mean streets; near-starved, unkept, and uncared for--they haven't known a good thing for a while! Such an existence can break a weak-spirited pup, leading him into a literal 'dog-eat-dog' mentality where he, eventually, trusts no one and may even be inclined to bite a feeding hand. You can quick-fix his contaminations and update vaccines, but if his temperament has been infected, You may find You've wasted time trying to rehab a mutt that will run again, or could even hurt You. This type of stray is more trouble than it's worth and is best left alone; assume he'll continue on as before. surviving on occasional treats from garbage cans, and look the other way.
Of course, there's no harm in keeping a box of treats handy for that unowned, familiar neighborhood pup who sometimes pauses his journey to visit for a pat and a rest at Your feet. He might even endear You with a trick he learned from a former owner! Still, it's better to observe a street dog's behavior for a period before attempting to affix a collar around his neck. There should never be a hurry to invite a mutt's fleas into Your house for any amount of time, until You are sure that his behavior is acceptable and the required loyalty is present.