Different microorganisms can cause distinctly different diseases, and the processes by which they damage the organism are different. We are all familiar with chronic infectious diseases such as tuberculosis, hepatitis, AIDS, warts, leprosy, and urethritis, so why do they persist clinically? Many microorganisms cannot be completely cleared by the immune response, but most of them are not lethal to us humans. These microbes have persisted in populations for hundreds or even thousands of years, are highly evolved, and have reached a state of compromise with the human immune system. They have learned to live with us humans, they are dependent on us and use us humans to survive. For example, gonococci have many variable antigens, such as bacterial hair proteins, which are encoded by several different genes, only one of which can be expressed. The silenced gene from time to time replaces genes activated downstream of the bacteriorhodopsin promoter. Thus, it can help gonococci evade specific immune attack. The pale spirochetes that cause syphilis have our human molecules encapsulated on their surface, and also these molecules are shed only when they infect tissues, which prevents the development of effective humoral immunity. HPV, which causes warts, on the other hand, is virtually incapable of generating an immune response because it is difficult to elicit recognition by the body’s immune system.