❌ = high virulence. Produce sudden high mortality (~100%)
❌ = intermediate virulence. Cause mild disease with mortality confined to young birds
❌ = low virulence. Cause mild disease or inapparent infection
❌= a virus that has a preference for the viscera (organs)
❌= a virus that is able to infect nerve cells
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Viscerotropic
Viscerotropic