Why is AIDS considered to be a “syndrome” and not a disease?

Syndrome is a group of symptoms, signs, physical and physiological disturbances that are due to a common cause. AIDS is also a complex of diseases and symptoms which develop due to failure of the body to fight off even minor infections. HIV that causes AIDS damages immune system of the patient by destroying T₄ helper cells (T lymphocytes). As a result, even small cold leads to development of pneumonia, a slight gut infection leads to severe diarrhea and blood loss, likewise skin rashes develop into ulcers. So it is a disease that causes all other diseases.