Toenail fungus is also known as onychomycosis. This appears when fungal infections appear underneath the nail, causing it to change in color and shape and at times pain and discomfort. Dermatophytes, the group of fungi, that causes athlete’s foot is the most common cause of nail fungus infections. Toenail fungus is more serious than just altering the appearance of the nail. In some cases, the fungi make it hard to walk and even wear shoes.