Immunity consist of several mechanisms evolved to protect host organisms against infectious diseases. Microorganisms use different strategies to evade immune responses of the host. The immune system must be able to combat all invading pathogens and their virulence factors. The observed differences in the effectiveness of immune response to various infectious diseases reflect differences between individual infecting pathogens. Immune mechanisms protecting against intracellular bacteria differ from those protecting against extracellular bacteria. Specific protective mechanisms are involved in immunity against parasite and viral infections.