Can you point me to where in canon we learned this, please? This is not my impression, but of course it's possible I'm missing something.
There's a classification of grosslings into the infection vectors they may use
here. Since the one in question was obviously not a "breather", we're talking about its teeth, some other body part, or its spit having gotten under Tuuri's skin. She had a wound in her shoulder that had bled right through her jacket in the mere seconds it took Lalli to get inside; if it had been only
spit that hit her (and the wound was caused by flying debris or somesuch instead), she would have removed the contaminated jacket as fast as she could, hoping to have it off her before the germs make it to the inside. So, we can take for granted that Tuuri had actually some troll in her flesh.
Given that, we can apply the knowledge that was stated about the consequences of direct contact between grossling and the wounds of a non-immune.
Here Mikkel suggests that infection through contaminated significant wounds is at least as likely as through a non-masked, breathing mouth - and that's the vector that got most of pre-Rash humanity killed.
Here Tuuri suggests that her entering Mora without a quarantine period could, in her opinion, get the whole of Outer Mora killed - and that's though she hasn't been near any contamination throughout the trip from Keuruu. In the same vein, she
later suggests that outbreaks tend to sweep entire villages. Siv and
all her colleagues - surely there are immune among them? -
work in full hazmat suits, even though the grosslings sealed in those glass cylinders should be
dead and, thus, have turned noninfectious before they ever got into the lab. The crew of
Túnfiskurinn was scheduled to
go into quarantine upon return to Iceland, in spite of them not being allowed to leave the ship - in Reynirs and Ólafurs case, the cabins - before that in the first place.
No, it's not like anyone has said "on screen" that you have x% of probability of infection per sq in of wound surface left by a troll, or that even a pin prick, if only deep enough, is a death sentence. But their actions speak reams, I'ld say.
