Every infection consists of a battle between the invading pathogen and the resisting host. To be successful, a pathogen must escape the many defenses of the host immune system until it can replicate and spread to another host. A pathogen must prevent one of three stages of immune function: detection, activation, or effector function. Examples of disease-specific immune evasion and the mechanisms used by pathogens to prevail over their hosts' immune systems are discussed. Also considered is what these host-pathogen interactions reveal about the normal function of the immune system and basic cell biological processes, such as protein maturation and degradation.