Several causes. There are many types of hair loss. For age-related baldness, a doctor may use rogaine (minoxidil) or propecia. For other causes, hair may grow back after the disorder is resolved: over-brushing, hormonal imbalances, stress, chemotherapy, autoimmune disorders, infection, etc... A doctor can evaluate for these other causes of hair loss. Normal people may lose 50-100 hairs/day, which are replaced by "new" hairs.
Many causes. The most common cause of hair loss is a condition called “androgenetic alopecia.” this condition is also known as “male-pattern baldness.” but that term is confusing, because the condition affects both men and women equally. Other causes include telogen effluvium, alopecia areata, tinea capitis etc.
Solutions available. Male pattern hair loss is mostly genetic in cause. It is passed down through the mother's family but the amount of hair loss can vary within the same family from mild to severe. Fda approved drugs Minoxidil and Propecia (finasteride) have been proven to stop hair loss and promote mild growth. Hair transplantation is effective in redistributing hair to the hairline and crown.