Foot pain: Over time, a nail infected with fungus can become thick and misshapen, causing pain, Dr. Joseph says. It can even make it difficult to walk when wearing shoes, according to the American Podiatric Medical Association.

Spread of the fungus: Sometimes untreated toenail fungus can spread to the surrounding skin on the foot. This may result in the athlete’s foot, a condition marked by itchy, red, cracked skin. “If you wear shoes and socks all day and one of your toenails is infected, it can spread quite easily because fungus thrives in a warm, dark, moist environment. 

The fungus can also spread to the genitals, where it becomes jock itch, a condition that can affect both men and women, according to the American Osteopathic College of Dermatology.

Widespread infection: This complication of toenail fungal infection is of particular concern for people with diabetes and other chronic conditions that weaken the immune system. If a toenail fungus spreads to the skin and causes it to crack, bacteria can get in. This may cause cellulitis, a condition that produces swollen, red, tender skin and must be treated with antibiotics, according to the U.S. National Library of Medicine. In severe cases of cellulitis, the infection can enter the bloodstream and be deadly. “And it can all start from a simple fungal infection.