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Histology, microscopy, anatomy and disease
Histology, microscopy, anatomy and disease

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1.1 Infection

Infection can affect any tissue of the body, producing cell damage and inflammatory reactions. There are many microbiological agents that cause infection, but two of the most common are viruses and bacteria.


Viruses are generally too small to be seen with the light microscope. However, their presence can often be inferred by the changes they produce in tissue, even if their identity requires confirmation by IHC, serology or molecular biology.


Bacteria can be seen with the light microscope using high magnification objective lenses, but the numbers of bacteria that are present in a tissue can be highly variable – even in one disease. A classic example of this variability is leprosy, where there may be very large numbers of bacteria in the skin (lepromatous leprosy), or very few (tuberculoid leprosy), depending on the precise nature of the infection.

Distinguishing the type of bacteria in a thin section of a lesion generally requires specialised histological stains, although the morphology (shape) of the bacteria may also be informative, as can be seen in the micrograph below.

Described image
Figure 2 Gas gangrene in a muscle. The micrograph shows a colony of bacteria, stained with haematoxylin. The bacteria have the characteristic shape and growth pattern of clostridia. Scale bar = 40 μm.

As with viral infection, the histological findings are an adjunct to serology and microbiology in producing a diagnosis, rather than a stand-alone explanation.

Activity 1

Based on what you learned earlier in the course, can you identify what type of infection can be detected in the blood smear?

Described image
Figure 3 (Scale bar = 20 μm)


This is a blood smear from a patient with a malarial infection. Several of the erythrocytes are infected. Malarial infection of blood was shown in the ‘Week 1’ slide set in the virtual microscope.