Why is the bird flu that has us all so worried not transmitted human to human (yet)? The virus doesn’t bind to receptors in the upper respitory tract (i.e. nose and throat) and instead binds to receptors deep in the respitory tract (think lung, specifically alveoli). So scientists will now start monitoring for any changes in the virus’s ability to latch on and invade upper respitory tract cells, which would mean easy human to human transmission. Whether such a shift is detected in the lab before people start dropping like, well, birds, is more than an academic question.