Pathogenicity represents a form of specialization that enables certain microorganisms to replicate within specific animals and damage host cells. The outcome is as dependent on the host as it is upon the properties of the pathogen. The ability of the human body to prevent most of the bacteria it encounters from doing harm is the result of an evolutionary course that has produced a complex set of overlapping defenses. The non-specific defenses include antibacterial substances such as complement, phagocytic cells, and the washing action of fluids such as saliva and urine. The specific defenses are cells producing antibodies upon stimulation, and cytotoxic cells. The non-specific defenses are the host's only defenses in the critical early period when infection develops, thus it is not surprising that many of the characteristics allowing certain types of bacteria to cause infection are characteristics that allow them to evade the non-specific defenses of the body. They include factors that help the bacteria to adhere to and invade cells and tissues. Some bacteria are well equipped to evade the body's defense mechanisms, and some produce toxins that cause symptoms and disease. The production of virulence factors is finely tuned and regulated.