Host-Pathogen interactions take place between a pathogen and their host, Host/pathogen interaction is emerging as a key area of infectious disease research in the post-genomic era. The ability of a pathogen to overcome host defenses can be accomplished by two distinct components: a primary pathogen or opportunistic pathogen. Infections are one of the major selective pressures acting on humans, and host-pathogen interactions contribute to shaping the genetic diversity of both organisms. Pathogens possess the inherent ability to cross anatomic barriers or breach other host defenses that limit the microbes that make up our normal flora. A significant part of human evolution has gone into developing ways to thwart microbial intrusion. Complex interactions between symbiotic or pathogenic microbes and the hosts they colonize are essential to both health and disease. Ongoing interactions between host and microbe have different outcomes that depend upon regulation of the host-microbe relationship and co-evolution of host and microbe favors an outcome. This rapidly advancing field is now bearing the results of interdisciplinary efforts by the researchers in the field of microbiology.