Immunization is undergoing important changes, with improved vaccines replacing less immunogenic or less safe vaccines, new vaccines for common diseases such as chickenpox and hepatitis A infection, and improved immunization schedules. Immunization is also being transformed by basic work in molecular medicine. Vaccines made of DNA are being developed as a form of gene therapy that use the patient's own cellular machinery to make foreign proteins that stimulate an immune response. Currently immunization is used to protect patients prior to exposure to an infectious agent or during the incubation phase after exposure, but before disease has occurred. New technologies are being investigated to induce the immune system to fight infections that have already produced chronic disease such as acquired immunodeficiency syndrome and chronic hepatitis B virus infection.