Chlamydiae alternate between two cell types called elementary bodies and reticulate bodies. The elementary bodies are released from infected host cells and enter uninfected host cells. In the newly infected host cells the elementary bodies transform to reticulate bodies. The reticulate bodies divide in the host cell and then transform themselves into new elementary bodies. The elementary bodies never divide and the reticulate bodies never invade host cells, they are both incapable of doing the other"s "job".

             The morphology and metabolisms of viruses are completely different from that of bacteria. The herpes group of viruses consists of a central core called a nucleoid, containing the viral DNA. The nucleoid is surrounded by a capsid made of tubular protein subunits called capsomeres. An envelope coated with viral antigens surrounds the capsid. Other viruses have variations of this morphology. .

             In the sense that chlamydiae change form between infecting and multiplying they can be compared to viruses. Viruses have extracellular and intracellular forms. In the extracellular form the virus is in the form described in the previous paragraph. When the virus infects the host cell it leaves behind its capsid and envelope so that only its nucleic acid enters the host cell. The viral nucleic acid is replicated by host cell machinery. So both chlamydiae and viruses, including the herpesviruses, have an extracellular form that attaches to the host cell and an intracellular form that replicates or is replicated in the host cell.

             The first step in the host-parasite interaction is the attachment of the parasite to the host cell. Chlamydial cell walls resemble those of gram-negative bacteria except that the chlamydial cell walls lack peptidoglycan. Instead of the peptide cross-links in the peptidoglycan layer, disulfide bonds between outer membrane proteins provide rigidity to the wall.