Hair loss is considered a symptom of an underlying disorder rather than a disease itself. According to the MayoClinic®, hair loss can be a sign of many things, including but not limited to: heredity, hormonal changes, medical conditions or medications. 
The causes of hair loss may vary depending
on the gender of the patient
In men, it may be the result of an imbalance in the production of the male sex hormone called testosterone. This condition is called male pattern baldness that tends to run in families, signs of this may be, gradual thinning on top of head or circular or patchy bald spots.
Then common causes of hair fall in women include hormonal changes and premenstrual syndrome. It may also affect women during pregnancy and menopause.
Other than these, lack of personal hygiene and local scalp and skin conditions can also contribute to hair loss.