You can cure a disease with one thing and it can cause another. Mercury was used as a medicine with success also but now we know better.
It's an unknown really. I think those who have used it for a short time in low doses don't have anything much to worry about, but that doesn't make it the best thing to use
Get it past your digestive system as with IV and you may well end up with a worse problem. Remember the woman who shed all her skin?
Externally, yes that could be the only safe use but even there it strips the skin of it's natural flora, which is an important defence, so it shouldn't be used as a preventative. Iodine would be my prefered choice for that too