The HIV virus weakens the immunity of a person because it destroys the lymphocytes. Human immunodeficiency virus (HIV) is a virus that causes the disease called acquired immunodeficiency syndrome (AIDS). The virus attacks a specific type of immune system cell in the body, known as CD4 helper lymphocyte cells. Lymphocytes are responsible for keeping us healthy from the common diseases and infections. HIV destroys these cells, making it harder for our body to fight off infections.