Why do my fingernails curl under?

According to the Toronto Star, fingernails that curve downward are sometimes called clubbed nails, a condition which is either congenital or a symptom of a developing condition, such as respiratory disease. Wikipedia notes that nail clubbing almost always involves enlarged and swollen fingertips around which the nails curve.

Wikipedia notes that finger clubbing or downward-curving fingernails is most commonly associated with lung disease, heart disease, gastrointestinal abnormalities and certain autoimmune disorders. The Toronto Star notes that some people are born with clubbed fingers, but those who develop the condition later on are likely exhibiting a symptom of another underlying disease.