Hair disease
Hair diseases are disorders primarily associated with the follicles of the hair.
A few examples are 

Alopecia
Bubble hair deformity
Hair casts
Hair loss
hypertrichosis
Ingrown hair
Monilethrix
Premature greying of hair
Pattern hair loss
Trichorrhexis invaginataMany hair diseases can be associated with distinct underlying disorders.
Piedra are fungal diseases.
Hair disease may refer to excessive shedding or baldness (or both). Balding can be localised or diffuse, scarring or non-scarring. Increased hair can be due to hormonal factors (hirsutism) or non-hormonal (hypertrichosis). Scalp disorders may or may not be associated with hair loss.

See also
List of cutaneous conditions

References
External links

https://www.nlm.nih.gov/medlineplus/hairdiseasesandhairloss.html