What causes fingernails to develop gray nail (onychomycosis)?

The main causes of onychomycosis (fungal nail infection) include: 1. **Fungal infection**: Onychomycosis is primarily caused by fungal infections. Common fungi include dermatophytes such as Trichophyton rubrum, yeasts such as Candida, and other molds. These fungi spread through direct or indirect contact, such as sharing slippers, towels, or other personal items, leading to infection of the nails. 2. **Weakened immune system**: When an individual's physical condition is poor and the immune system is compromised, the body may be unable to effectively resist fungal invasion, thereby increasing the risk of developing onychomycosis. 3. **Poor hygiene**: Neglecting personal hygiene, staying for prolonged periods in damp and unhygienic environments, or wearing non-breathable shoes and socks can all promote fungal growth and subsequent nail infection. 4. **Trauma**: After nail injury, skin sensitivity increases and resistance decreases, making it easier for fungi to invade and cause onychomycosis. 5. **Underlying medical conditions**: Certain diseases, such as diabetes and peripheral nerve disorders, may increase the likelihood of developing onychomycosis.