I'm not a doctor, but I did go to Barber School. Here is what my barber book (Milady's Standard Textbook of Professional Barber-Styling) says...
Alopecia senilis is baldness occurring in old age. The loss of hair is permanent.
Alopecia prematura is baldness beginning any time before middle age. It occurs by a slow, thinning process, and is due to hairs that fall out being replaced by weaker ones.
Alopecia areata is the loss of hair in round patches, or bald spots. It can be caused by anemia, scarlet fever, typhoid fever, or syphilis. Affected areas are slightly depressed, smooth, and very pale due to decreased blood supply. Patches may be round or iregular, and vary in size from one-half inch to two or three inches in diameter. In most cases of alopecia areata, there has been injury to the nervous system, the affected area is usually poorly nourished as well.
Alopecia may appear in different forms, resulting from a variety of abnormal conditions. Sometimes alopecia can be treated successfully by proper scalp treatments.
Hope this might help. I typed this out fast, so I hope I got it all, and typed it correctly.