Principles of Diagnosis of Infectious Diseases

Infectious diseases are diagnosed by detection of a bacterium, virus, fungus, protozoan, or helminth in a patient with a compatible clinical illness. The methods of detection include cultivation of bacteria and fungi on growth medium, isolation of viruses in cell culture, and identification of the agent biochemically, antigenically, or genetically. Infectious diseases can also be identified by detection of a specific immune response, usually antibodies, that develop during the course of illness. Visualization of an agent in infected tissue can provide a diagnosis based on specific morphological characteristics or identify the category of organism, for example, gram-positive or gram-negative bacterium or virus (e.g., eosinophilic cytoplasmic inclusion bodies in neurons in rabies virus infection). Methods that detect and allow visualization of antigens (immunohistochemistry) or nucleic acid sequences (in situ hybridization) provide more specific diagnoses. Detection of specific nucleic acid sequences amplified by polymerase chain reaction is a powerful molecular diagnostic tool. Since the elucidation of the etiology of the first infectious disease, anthrax, more than 135 years ago, Koch's postulates have been applied and modified as novel technologies and agents have emerged. During the last 45 years, more than 70 previously unknown agents of infections have been identified in emerging infectious diseases, a phenomenon that is likely to continue.