Hi Char-x
There are some pictures in these pages of piggies with mites, and fungal, some have both at the same time when they arrive as rescues. They look very similar in the early stages, but the big giveaways with fungal are if there is mild to heavy dandruff, if the skin feels hot (sometimes if your pig has areas of pink skin, these will appear more than a shade deeper than the normal pink behind the ears), and if you can tug gently on a couple of hairs in the suspect area and they come out easily, and often have a little clump of skin attached. The Gorgeous guineas page is the best I've found for identifying and comparing symptoms.
THISTLE CAVIES LATEST RESCUE PIGGIES UPDATE LATEST RESCUE PIGGIES.
Fungal in the UK rarely looks like the pictures on GL. It tends to occur mostly on the mid to lower back at first from what I can gather and spreads from there. Having said that it can occur on the face. As for how it got there, it's just one of those things I'm afraid. We have a damp climate and there are mould spores everywhere in the air around us, sometimes they get settled in a piggy's coat and set up an infection. It's unavoidable, but you can minimise problems by bathing every 6-8 weeks with gorgeous guineas shampoo to remove them, or your normal piggy shampoo. Last summer was dreadful for fungal because it was so damp. Some people use disinfectant in the cage regularly, but I don't think that it's good for the piggies, and it's effects will be limited. It is often used by people with wooden outdoor cages because those cages become damp with urine and start to rot if they aren't sealed properly. Their piggies shouldn't be living in that state anyway. Indoor piggies are less prone to fungal, but humidity still builds up from drying washing, showers etc.
As far as treatment goes, very few vets can diagnose fungal, but if your vet is prepared to learn, Gorgeous Guineas are very effective. Rescues use them, and the details are on the website. However you could also use human anti-dandruff shampoos like Nizarol, T gel and head and shoulders. I think you let the shampoo soak in for about five minutes and use them about a week apart for three weeks, but I'd have to check that. You also have to do a patch test, by dabbing a little shampoo behind your piggy's ear about 48 hours before you want to bath them to check they aren't allergic. I would bath all your piggies, as fungal can spread once it's on a piggy.