Toenail fungus onychomycosis is caused by a group of fungi known as dermophytes this group thrives on skin and on keratin the main component of hair and nails the fungus gets under the nail. Foot fungus usually affects the skin as well as nails so it is important to treat both when combating fungus fungus does not go away on its own so if left untreated it is contagious and will spread you should check your nails and skin regularly some signs of fungus are yellow or white nails that are usually very thick and break off. Fungal and nail infections no one likes to talk about it but many people experience fungal and nail infections read on to see how to avoid these types of infections and what to do if you have an infection on your feet or toenails. Toenail fungus is a fungal infection called onychomycosis tinea unguium is another name for it its an overgrowth of fungi that are often found on your body together with other bacteria you can get the fungi from someone else too like at a nail salon if the tools arent properly sterilized its the same fungi that cause ringworm jock itch or athletes foot