So, fungal spores are literally everywhere, and the requirements for fungus to thrive seem to be trivially low; give it a moderately humid environment and it’ll grow on a bare concrete wall ffs eating god only knows what; the dust from the air maybe?
Well, and the great outdoors is full of slightly damp places, many of them downright soggy most of the time - and absolutely rife with organic material to snack on.
Where’s the bottleneck? Why isn’t the world a choking fungal hellscape?
Mold needs the proper conditions to grow.
Probably most important to your question is nutrients. Mold needs certain nutrients in order to grow, which is why it doesnt grow on everything.
Mold also need decently specific moisture in its evironment, a proper temperature range, shielding from UV rays that kill it, and if these conditions are met, mold can grow in about 24-48 hours.
Why don’t arse cheeks get moldy?
Because you wash them often enough that they don’t get moldy, hopefully.
Yes, you can actually have mold grow on your skin. But there are a lot of bacteria competing for food with the mold, as well as body oils and waste secretions, on top of the body’s immune system being pretty good at taking care of invasions.
There are quite a few different types of fungal skin infections (athlete’s foot, ringworm, fungal nails, an array of yeast infections), some can be pretty damn persistent!
Your body’s immune system may be pretty good, but fungal infections are some of the most difficult to fight. For example, if you get fungal meningitis, it’s a massive headache to deal with.
You also constantly shed skin, so any mould that tries to get a hold would have a difficult time staying on.
They definitely can