*Most mice show no symptoms due to maternal antibodies
*Most mice show no symptoms due to maternal antibodies
*But minor respiratory lesions may invalidate carcinogenic or toxicological studies
*But minor respiratory lesions may invalidate carcinogenic or toxicological studies
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*Immunological studies also confused due to virus activating NK cells via high circulating IF 3-4 days post-infection
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*Immunological studies also confused due to virus activating [[General Pathology - Antigen Recognition, Processing and Presentation#NK Receptors|NK cells]] via high circulating IF 3-4 days post-infection