Yellow nails certainly don’t look pretty, and what causes the hue is even grosser: “Thickened nails, with or without a yellow-ish tone, are characteristic of fungal infections that generally traverse the entire nail bed,” Dr. Agarwal says. She adds that topical medication is often no help since the infection is in the nail bed and underlying nail plate. Your doctor can prescribe an oral med, which will reach the entire breadth of the infected nail.