Infectious diseases obviously affect us; our personal health, and the personal health of other people. If someone else has an infectious disease then you would probably stay away from such a person. If you had an infectious disease, other people would probably stay away from you. This isn't true for noninfectious diseases where we don't have to worry about infecting or being infected by other people.
Because the brain stem is what transfers all information to the rest of your body. Also, it is directly connected to the hypothalamus, which if damaged, can take away your ability to breathe or do simple tasks.
