While oncology remains the dominant application, Immunohistochemistry holds a significant and growing role in the diagnosis of Infectious Diseases, particularly when conventional methods like culture or serology are inconclusive or difficult. IHC is highly effective for identifying microorganisms that are present in low numbers, are fastidious (difficult to grow in a lab), or exhibit an atypical morphology that is hard to distinguish using routine hematoxylin and eosin (H&E) staining.

By using specific antibodies directed against bacterial, viral, fungal, or parasitic antigens, pathologists can precisely localize the pathogen within the infected tissue. This is especially useful for diagnosing viral infections such as Cytomegalovirus (CMV) or Herpes Simplex Virus (HSV) in biopsy samples, even before characteristic cytopathic changes are evident. The high specificity of the antigen-antibody reaction allows for a definitive identification that can quickly inform clinical treatment, proving invaluable in complex cases, especially those involving immunocompromised patients.