Toenail fungus is caused by fungal organisms, primarily a type of mold called dermatophytes, which require keratin (a protein in nails) to grow. Other fungi such as yeasts and molds can also cause nail infections

. The fungi typically enter the nail through small cuts, cracks, or separations between the nail and nail bed, where they find a warm, moist environment ideal for growth

. Common causes and risk factors include:

• Having athlete's foot or other fungal skin infections, which can spread to the nails

• Walking barefoot in damp public places like swimming pools, gyms, and showers where fungi thrive

• Wearing tight, closed-toe shoes that cause sweating and moisture buildup

• Older age, as slower nail growth and reduced blood circulation increase susceptibility

• Minor skin or nail injuries that allow fungal entry

• Health conditions such as diabetes, weakened immune system, poor circulation, or psoriasis

• Frequent exposure to wet or moist environments and poor foot hygiene

In summary, toenail fungus develops when fungal organisms infect the nail through breaks in the skin or nail, especially in warm, moist conditions, and is facilitated by factors such as athlete's foot, damp environments, tight footwear, age, and certain health conditions