Pathogenic mycoplasmas have been found in the blood or other specimens of patients with a variety of chronic clinical conditions including respiratory, oral cavity, genital and other infections, autoimmune, inflammatory and immunosuppressive diseases, and fatigue syndromes of unknown origin. These small bacterial microorganisms are possible causative agents, cofactors, or opportunistic infections in these and other illnesses. Evidence for their association or possible role in various clinical conditions is suggested by their significantly higher incidence or degree of infection in symptomatic patients than in non-symptomatic controls, and their gradual suppression by the appropriate antibiotics resulting in gradual patient recovery from clinical signs and symptoms. Although they are not widely appreciated for their pathogenic properties, certain Mycoplasma species and certain other species of bacteria ((Chlamydia, Borrelia, etc.) appear to play a role in disease progression or patient morbidity in rather large subsets of chronic illness patients.