B, AIDS is a viral infectious disease, and it is a disease that human beings are infected with human immunodeficiency virus (HIV), resulting in immune deficiency and loss of human immune function. People infected with HIV and AIDS are the only sources of infection of this disease. Therefore, AIDS is highly contagious, but contact with AIDS patients or working together will not be infected. B correct;
C. Immunity is a defense function of human body. Through this function, the human body can identify the "self" and "non-self" components, destroy and repel the antigen substances that enter the body, remove the damaged nuclear tumor cells produced by itself, and maintain the balance and stability of the internal environment of the human body. So it is correct that immunity has a protective effect on human body. However, when some organs of the human body are damaged and need to be transplanted, it is not easy to transplant because of immunity. In this case, immunity is not.
D, when taking medicine, we should not only look at the function and purpose, usage and dosage, expiration date in the instructions, but also pay attention to the adverse reactions of drugs. D is correct.
So choose: C.