18 month old and stranger danger?

My daughter is almost nineteen months, and we cannot stop her from screaming her lungs out whenever someone unfamiliar is close to her or even in the same room. Aunts and uncles, she screams at and becomes hysterical. Doctors and nurses and friends and random employees.... If we're in an open area like a store, she's okay. But if we're in a hallway or room with someone she freaks out and won't stop until they go away. Is this normal??? Advice?? How long will this last??