Infectious Diseases: Definition, Transmission, and Diagnosis

Definition of Infectious Diseases

An infectious disease occurs when a parasite develops and multiplies within a host. The causal agent of transmissible diseases is usually a living organism that moves from one individual to another by contagion. The set of elements that define a communicable disease consists of:

• Reservoir: Where infectious agents are usually found.
• Source of Infection: Animate or inanimate object from which the causative agent originates.
• Mechanism of Transmission: The pathways that allow the causal agent of a transmissible disease to contact the host.
• Host: A person susceptible to contracting a communicable disease upon contact with the source of infection.

Samples arriving at the laboratory for the diagnosis of causative agents will be sent to:

• Microbiology: For identification of the infectious agent.
• Tissue Culture: To study invasion and multiplication of the agent.
• Anatomic Pathology (AP) Lab: Routine procedures include buffered formalin fixation. In cases of spongiform encephalopathy, samples are treated with 96% formic acid.

Factors Favoring Infectious Diseases

Increasing global population mobility (tourism, immigration) has altered the geographic distribution of infections. Immune states, whether resulting from natural disease (e.g., AIDS) or iatrogenic causes (due to medical intervention), play a role. Adaptive microorganisms can jump species barriers and explore new environments.

Microorganism Detection and Identification

Patient's Clinical Presentation

In most cases, diagnosis will not be possible without tissue samples. When a study in Anatomic Pathology is performed, it is important to have comprehensive data on the patient's clinical history, especially details of ethnicity and immunological status.

Gross Appearance of Tissue

The presence of abscesses, pus, necropsies, focal granulomas, etc. These lesions are nonspecific but may be diagnostic, as in the case of hydatid cysts or helminthic manifestations. When a portion of tissue is studied, it is sent to the microbiology lab, another to the tissue culture lab, and another is fixed and included in the macroscopic section.

Microscopic Appearance and Routine Stains

Routine stains often provide indirect evidence, such as neutrophil or lymphocyte infiltration, granulomas, etc. Some of these findings may be sufficient for an initial diagnosis.

Specific Staining Techniques

Special staining techniques, such as the Gram method for bacteria, Grocott method, fluorescence methods, and methods derived from hematoxylin or fuchsin stains, are used to increase the visibility of different organisms. Immunohistochemical and immunofluorescence methods identify microbial antigens in tissue using labeled antibodies.

Electron Microscopy

Electron microscopy is performed for the study of viral diseases.