I think z's probably eat more lizards than anything else in the wild so a diet of lizards, or at least including lizards in their diet, probably improves overall health and immune system function. If z disease is something that alot of z's carry naturally(and I'm leaning towards that conclusion) then having a healthy immune system probably does go a long way towards preventing z disease. What's wierd about z disease is that once it blooms in one animal it seems so highly contagious. Opposed to something like mouthrot where you certainly have to be careful and practice normal hygiene but mouthrot doesn't just miraculously hop from cage to cage the way z disease seems to.
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