The characters in John Carpenter's the thing are not intelligent.
Last night I rewatched The Thing for the dozenth time and caught something I never did before. When Macready asks them to draw blood the same knife is used on everyone. Windows extracts blood from Nauls and then wipes the blade on his pants before using it on himself. It's quickly established that even a molecule from the thing would infect a person. So when they are drawing blood to test who the thing is they are infecting every non-infected person. #conversation