The anatomic pathologist performs an important role in the diagnosis or exc
lusion of infectious diseases. The morphologic interpretation of biopsies a
nd cytologic preparations allows for the definitive establishment or exclus
ion of a wide variety of diseases. Once the pathologist has determined that
a disease is likely to be due to an infection and has characterized the in
flammatory response, associated microorganisms or viral-associated cytopath
ic effects should be recorded. Although some microorganisms or their cytopa
thic effects may be clearly visible on routine hematoxylin and eosin-staine
d sections, additional histochemical stains are often needed for their comp
lete characterization. Highly specific molecular techniques, such as immuno
histochemistry, in situ hybridization, and nucleic acid amplification, may
be needed in certain instances to establish the diagnosis of infection. Thr
ough appropriate morphologic diagnoses and interlaboratory communication an
d collaboration, the anatomic pathologist contributes greatly to the diagno
sis and treatment of infectious diseases.