Causes of nail fungus?

From reading many messages on this forum it seems that the cause of nail fungus is the wearing of shoes constantly and never allowing the feet to breathe.

I'm not sure i buy that to be honest as i've lived for years wearing mainly open-toed house-shoes and nail fungus has got worse. How often i wash my feet doesn't seem to change the condition either.

I'm really not sure it's to do with 'outside' causes of lack of hygiene, or wearing closed-shoes too much. If that was the cause then why doesn't EVERYONE suffer with this as most people wear closed shoes?

Personally i think it's to do with an inner-body fungus problem that is becoming physically symptomatic on the outside as a sign that the inner problem is BAD!

If i treat with oils, CS etc the nail will begin to grow fresh - but stop the potent antifungals - and the problem is back!

Anyone else have any insight into the cause? Is it just external fungus that we spend the rest of our lives treating...or is it internal fungus showing up on the outside?