Let's look into the life of an infected person. See how it is to live day to day, deal with the same circumstances we deal with everyday. Now imagine this person has no distinctive characteristics that would allow us to determine if he is indeed infected. Imagine this infection is not at all superficial yet once you get it you can't get rid of it. Now imagine, for a moment, that you are this person, you have this infection and now you've set out, on a mission, to deliberately infect whomever you come into contact with. And now imagine the terror you would feel knowing you have actually infected some one close to you with your disease? This begins your quest, a means to an end, to dealing with your infection.