Final answer:
Alopecia areata is the medical term for the sudden loss of hair in round patches or spots, caused by the immune system attacking hair follicles. The hair growth cycle includes the anagen, catagen, and telogen phases, and disruptions can lead to hair loss.
Step-by-step explanation:
The term for the sudden falling out of hair in round patches or baldness in spots is called alopecia areata. This condition can cause hair to fall out quickly on the scalp and sometimes other parts of the body in smooth, round patches. It occurs when the immune system mistakenly attacks hair follicles, which is where hair growth begins.
The hair growth cycle consists of three phases: the anagen phase, which is the active growth phase of the hair; the catagen phase, a transitional period; and the telogen phase, the resting phase before the hair falls out. If more hairs enter the telogen phase than normal or the anagen phase is disrupted, excessive hair loss may occur. While alopecia areata can be distressing, it is usually not permanent, and hair may regrow over time.