This article discusses all things mentioned here, and I thought this was interesting, since nail-salon pedicures are so popular these days:
"There’s no one route of [fungal nail] infection, but sources include pedicures and locker room showers. Fungi love moist, dark environments such as wet shoes, Kominsky says, so people such as lifeguards, landscapers and fishermen are particularly susceptible. Shoes are the reason why toenail fungus is much more common than fingernail fungus, which can also occur.
The elderly are more likely to have toenail fungus than younger people because….."
New laser treatment takes aim at toenail fungus - The Washington Post