is this something that if treated goes away for good, or is it that once contracted the dog has a propensity to get it multiple times?

Ringworm is a fungal infection that can be contracted by anyone (or any dog) at any time.

Def treat as soon as possible and yes anyone in the home can get this since it is very spreadable. An you must treat your place as well by cleaning everything and washing everytime you have to put the ointment on ur pet or yourself.

http://www.publichealthgreybruce.on.ca/communicable/Fact-Sheets/Ringworm.htm

http://www.pets.ca/encyclopedia/ringworm.htm

My friends cat she bought had ringworm and it was passed onto her other cats and children b4 she even knew what it was- she ended up having to put her cats on pills to rid it- ended up being pretty costly in the end for her. Not to be mean but I never visited until I knew she was free of it.

My daughter a couple years back got ringworm from the school but luckily our pets never got it and we cleared her up fast with some sab stuff.

I caught ringworm when i was working in quarantine kennels/cattery, took ages to go ! It can also lay dormant in woodwork for years & years which is how the infection came about there & even though i wore protective clothing & only worked with the cats that caught it ,it did spread very fast to other cats in the block.
Edited to say... i was led to believe that once you've had it you are more prone to get it again ..... i've ot had it since & its been 14years ago but then again i'm not a cat or dog lol.