The implication is that the jotnar was patient zero for the rabies outbreak and infected all the other trolls. So how did a solitary troll living in Arctic mountains above the snow line get infected with rabies, let alone an enormous animal that has no natural predators? Norway has officially been rabies-free since the nineteenth century. Did somebody do it deliberately with an agenda?
An infected animal such as a wolf or a bat that got in from the Russian border could have infected the jotnar.
Considering the end of the film states that none of the teenagers were never seen again it's likely that Thomas never got to a hospital. Good luck with that rabies treatment.
Just where did Hans get that bucket of Christian men's blood?