Immunity is the condition of being able to resist a specific disease, particularly through means that prevent the growth and development of disease-carrying organisms or counteract their effects. It is regulated by the immune system, a network of organs, glands, and tissues that protects the body from foreign substances. Immunology is the study of the immune system, immunity, and immune responses. Progress in immunology over the past two centuries has made inoculation—the prevention of a disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes the disease—widely accepted and practiced. Despite such progress, however, some diseases evade human efforts to counteract them through medicine or other forms of treatment. This is particularly the case with a disease in which the immune system shuts down entirely: a condition known as acquired immunodeficiency syndrome, or AIDS.