>because the dead can't get sick, but if something never worked, being
>dead won't help...
Yeah, pretty much this. I've never set out to establish a definitive reason for it, because she's never investigated (she doesn't really care), but I suspect it's because Toph was born blind (as opposed to someone who lost her eyesight to injury or illness later in life).
I doubt there's anything inherently wrong with the quarian immune system at the species level - evolution doesn't work that fast. It's an environmental syndrome, and it doesn't apply in Valhalla. For that matter, there's nothing to catch there anyway.
Out-of-band, it's because her blindness is such an integral part of her character design that she's kind of not Toph without it. That may seem a little heartless, but there you are.
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