I think it's just a common misconception that the Thing can infect people on a microscopic level like a virus. If that were the case, Bennings would've been assimilated from the get-go after being licked on the mouth by the dog-thing, like three minutes into the movie. Mac and Doc would've been assimilated from picking up and carrying the split-face carcass on board their helicopter. Everyone in the blood test scene would've been infected by sharing the same scalple. Blair would've been assimilated from putting the pencil eraser in his mouth.
Also, if the Thing COULD infect through casual contact, it would have absolutely zero need to assimilate people by going around and painstakingly revealing itself over the course of a week. It could literally just put cells of itself on any place, like you said. Why would it choose to go days out of its way by doing the hard road?
I think that if a single Thing cell was inserted into the human body, the immune system would likely kill it. Sure, we do see that a single Thing cell can assimilate cells on a cellular level, but perhaps this can only be done in small environments, like in a petri dish. Humans have 39 trillion cells in the body, so for one single cell to attempt to overthrow 39 trillion cells would take a long time.
This is all speculation I guess. Toodles!
Good point. I agree!
If microscopic amounts of the thing could contaminate people, it wouldn't have needed to do the things it did. Then again, maybe the screenwriters just overlooked that fact.