Nails can signal all sorts of issues with the body, like autoimmune disease, diabetes, thyroid disease, etc. The most common direct cause is fungal infection. The appearance usually changes, and you didn't describe a color change.
A fungal infection may start as spots of brownish discoloration or the nail may thicken and get bumpy. It can get a little tender.
Use 100% tea tree oil on all the toenails morning and night. The bad nail has to grow out. Clean your tools for trimming the nails. Avoid keeping your feet in moisture. Wash socks and underwear in hot water and add baking soda to kill fungus. It spreads easily.
Get checked out for diabetes, thyroid problems, and circulatory problems. Check for food allergies.
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