In general, ringworm infections are very common, perhaps affecting as many as 20% of the population at any one time. The dermatophytes that cause ringworm thrive in damp, close environments and so infections are more common in geographical areas that are warm and humid. Ringworm infections tend to get worse during the summer months and may abate during the winter months.

Athletes are at higher risk of ringworm because heavy sweating favors the growth of the fungi that cause ringworm. Excessive perspiration washes away natural skin oils that kill fungi making the skin more prone to infection. People who are overweight are also more prone to some types of ringworm, including jock itch. Jock itch is most common in adult males.

Ringworm is common in young children, probably because it is so easily spread from person to person in an institutional setting like a school or child care center. Having pets increases the risk of a child contracting ringworm.

People with weakened immune systems, such as those with diabetes or HIV/AIDS, have an increased risk of ringworm, as do people with the chronic skin condition called atopic dermatitis.