A parasite is defined as an organism that lives in or on another organism called a host (2). If the parasite has the capacity to cause disease in the host then the parasite is called a pathogen. Disease in the host is caused by the infection of the parasite. The interaction between the host and parasite is complex. Both the pathogen and the host strive for survival in some of the cases. The pathogen divides within or on the host in an attempt to keep its species alive while the host"s defense mechanisms simultaneously attempt to eliminate the pathogen. The extent of the "battle" for survival varies depending on the relationship. This paper discusses the disease state of Chlamydia; how the organism invades its host, evades the host"s defense mechanisms, multiplies within the host, and is released from the host. Certain aspects of the chlamydiae will be compared to the other pathogens, Rickettsia and the Herpes viruses as they relate to the disease state.

             Bacteria are classified into four categories according to shared characteristics, these categories are then divided into groups, and the groups are divided further into subgroups. The ninth group of bacteria contains only two subgroups called the Rickettsias and Chlamydias (1). According to 16S r RNA sequencing Rickettsias are related to the purple Bacteria and Chlamydias comprise a major branch of Bacteria (2). Viruses are not grouped among the prokaryotes. In fact viruses are not really organisms by definition. They are genetic elements that are replicated by host cells. The herpesvirus group contains over seventy viruses, all of which are potentially pathogenic. Only five of these viruses infect humans. This group of viruses resembles each other and has biological properties in common, particularly the latency-reactivation stages in the disease state.

             Before discussing the host-parasite interactions the developmental cycle of chlamydiae need to be mentioned briefly.