
Continuity mistake: Nurse Jill turns around before Michael Myers stabs her. After she's stabbed, she's already facing forward and her head doesn't snap to face the camera.
Continuity mistake: After Nurse Karen is drowned by Michael Myers, he drops her backward. The close-up shows however that she falls forward.
Answer: I have read that, in early drafts of the script, the hospital was a health clinic, not a standard hospital. This would possibly explain why there are only a small number of patients, though it doesn't explain why there is a maternity ward, or why the mother brings her son there for emergency treatment.