When a toenail is ingrown, it is curved and grows into the skin, usually at the nail borders (the sides of the nail). This digging in of the nail irritates the skin, often creating pain, redness, swelling and warmth in the toe.

If an ingrown nail causes a break in the skin, bacteria may enter and cause an infection in the area, which is often marked by drainage and a foul odor. However, even if the toe is not painful, red, swollen or warm, a nail that curves downward into the skin can progress to an infection.