Anxiety. Anxiety symptoms can mimic many illnesses, such as heart or lung or intestines/stomach or neurological, as well as can intensify symptoms of underlying illnesses such as gastrointestinal or neurological. Thorough workup can clarify the illness.
Yes. Yes, anxiety can express as symptoms in nearly any organ system -- especially neurological, gi, pulmonary, and cardiac. Many people with panic attacks, for instance, fear they're having a heart attack and go to emergency rooms because of this.