The cause of dandruff is fungus, a bacteria that you have to treat. It is not contageous, and can be effectively treated by some topical solutions, like Luxiq, and shampoos like Loprox.
The dry scalp is caused due to environmental factors and chemical treatments used for styling the hair. The main reason for the dryness being the depletion of moisture level.
Oily scalp is caused by the overproduction of sebum, by the oil glands in the scalp. This could also be due to the inadequate rinsing out of the hair. See related links below.
from your old shampoo or washing your hair too much
Skin on the body undergoes constant renewal. This means that each and every skin cell, once it dies, gets pushed out and replaced by newer, healthier cells underneath.
Dandruff is essentially this process. Those with dandruff have skin on their scalp that sheds quicker than those without it, making the dead skin "flakes" a lot more noticeable.
Dandruff also occurs when the scalp is exposed to extreme temperatures and cause the skin cells on the scalp to dry out and shed quicker.
To prevent it, use hair products such as "Head and Shoulders" that are designed to treat dandruff. Basically, aim for anything that will keep the scalp healthy and moisturized.
dandruf is caused by dirt that has been decayed by not washing
your hair for many days
A common cause of dandruff is because of an incompatible dandruff or if the hair is not thoroughly washed.