Alopecia is the medical term for sudden hair loss. The hair loss that occurs with this condition occurs rapidly and in circular patches on the scalp. Some individuals also lose the hair on their eyebrows, eyelashes, beards, their private areas and even the underarms. Alopecia is common in both men and women, and while many might think it strikes only middle-aged or older people, it often appears before a young man or woman reaches his or her twenties. Medical researchers don't know what causes alopecia, but learning how to understand and identify symptoms of alopecia prompts may individuals to seek early treatment.