Sometimes toenails that curl up (or down) are caused by fungal infections. Fungal infections in the toenails are fairly common, especially in women and people who are older than 60 years of age, although anyone can get a fungal infection. It is also common in people who have decreased circulation.
Fungus thrives when people have warm and moist feet, so this is why it is important for people to keep their feet dry and clean. Fungal infections can also be contagious. It is always important to see a doctor in order to find out what is going on and to get proper treatment.