Nail Fungus
That fungus that makes your nails look so funky is extremely contagious. As the American Academy of Dermatology has explained, the fungus is very transmissible in warm, damp environments.
Even more than that, sharing your shoes with someone when you have that infection might also cause it to spread. When you have noticed that you have fungus on your feet, just make sure you avoid walking around barefoot.
You should be extra careful in general about wearing shower shoes at the gym or double-checking your friends’ feet before you decide to borrow a pair of shoes from them. You never know!