Rabies encephalitis, Negri body
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The cytoplasmic Negri body is pathognomonic (diagnostic) of rabies encephalitis. It is about the same size (7 micrometers) and color as a mature RBC. However, if you have to wait to make the diagnosis by brain biopsy, you are too late. Rabies can only be successfully prevented, not treated. Mortality is essentially 100%, once the virus reaches the brain.