• May be predicted from the requirement of the virus for rapidly dividing cells ( See Picture ).
• Main target organs are the crypt epithelium of the intestine, and lymphoid tissue and the bone marrow.

Intestinal effects

• Virus localises in crypt cells because these are most mitotically active.
• When crypt cells are destroyed by virus, no new cells are available to replace adsorptive cells at villus tips.
• Therefore factors influencing mitotic rate of crypt cells (e.g. presence of bacterial flora, fasting) to some extent determine severity of disease.

Lymphoid tissue and bone marrow effects

• Virus attacks lymphocytes in lymphoid tissues, and leucocytic stem cells in bone marrow, leading to panleucopenia.
• Erythrocytopoiesis appears to remain unaltered; red cells survive longer anyway; therefore generally in the acute case, no anaemia present.

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