Proactive behavior by an individual in psychology explains that the individual is trying to change behavior by various means i.e by setting future goals to do something to overcome from something, also referred to as self-initiated behaviors. In this example, Bill is trying to initiate a behavior( spending time with family) to deal with the stress he is facing.
When the body is given weakened/dead pathogenic material this helps build up an immune system towards the actual live infection. It prepares the body for the infection so that the person does not get sick.