Highlights of infectious agents in tissue.

The evolution of the diagnosis of infectious diseases began with the observation of the morphological characteristics of organisms such as ascaris and whipworms, followed by the use of the microscope and haematoxylin and eosin stains, which allowed recognition of microscopic characteristics undetectable with the naked eye, such as the viral cytopathic changes of herpes and the presence of fungi. Patterns of acute and chronic granulomatous inflammation were also observed; these were not specific to the exact aetiology of the disease, which led to the introduction of special methenamine stains for fungi and Ziehl-Neelsen for fungi and mycobacteria. Later, the use of immunohistochemistry was introduced, which acknowledged the use of antibodies to classify microorganisms and detect cases that were either difficult to interpret or in the midst of severe inflammatory processes. Currently, the use of molecular biology has made it possible to reach diagnoses that would have been very difficult to obtain through traditional methods; these techniques show key specific characteristics and facilitate the diagnosis of various infectious pathologies. These new techniques are based on the detection of antigens and nucleic acids of microorganisms, an important advance in the diagnosis of infectious diseases.