Thanks for playing! The screen shake means the object/person is NOT infected at that point of time, if it turns red, it means they are. I should add this to the instructions ><
Ideally, since the target can get sick from objects as well - testing an object before and after a "suspect" has come in contact with it can tell you if that "suspect" carries the infection.
Do you think this information makes the gameplay more strategic than luck based? I'd love to hear :)