I ended up fielding a question about HIV from someone. He had been largely ignoring the disease, and, when forced to face it, didn't know how to reassure himself that he wasn't infected, despite testing negative a few years ago. I told him how I stay sanguine about such things, a psychological defense mechanism which I learned during the years when my work involved rather ugly pathogens. I remember how much such things scared me as a child, so hopefully this will be helpful to someone else.
You fear a disease because you don't know whether you have contracted it or are likely to contract it, and because you have no idea if your actions are going to cause you to contract it or not.1 To reassure yourself on those counts, you need to do three things:
I'll take my approach to HIV as an example.
How is HIV transmitted? To find out about the transmission of this or any other disease the place to start is the Center for Disease Control, a US government agency tasked solely with researching diseases and providing reliable information to the public about them. They have a whole set of pages detailing HIV. The transmission mechanisms are sex or ending up with someone's infected bodily fluids mixing with your own, so I need to account for those two routes, and nothing else.
Every few of years I have my doctor run a standard set of tests for sexually transmitted diseases. The last one was about two years ago, and was negative. That gives me a point in the past when I know that I wasn't infected. Since then I have been in a monogamous relationship with my wife, who was also tested and found not to be infected at around the same time, so there is no source for me to get it via sex. Nor have I had a blood transfusion, gotten in a fight, or dealt with serious injuries since then. The only open wounds I have dealt with are my own and my wife's, so there is no mechanism by which I could be infected, and I am confident that I remain HIV negative.
Simple, yes? Of course, it's simple because my lifestyle puts me at a very low risk of infection by HIV. The precautions you need depend on your lifestyle. When I was working in a tuberculosis laboratory, we were tested twice a year for tuberculosis and maintained very strict containment protocols in order to meet these three points, though we all had nightmares every so often about containment breaches. Police and bouncers should be tested regularly for blood borne diseases such as HIV, since they are regularly around violence and the leaking bodily fluids violence often results in. And they should be tested again after particular incidents, such as being bitten by a human. Similarly, someone in an open marriage would need to take a different set of precautions than I do, as well, simply because the exposure routes are different.
But if you can specify those three points with confidence for a disease, it ceases to be frightening.2
1 For some reason cancer doesn't inspire the same fear in most people, I suspect because its onset is random. Its risk may be affected by your life history and (to a lesser degree) your genetics, but there will be no action you can point back to and say, "If I hadn't done that, I wouldn't be ill." Unless you're a woman of Ashkenazi Jewish descent carrying the BRCA1 mutation, in which case your birth was the event, and you should be looking into getting a mastectomy.
2 Though there are diseases that you can't do so for. The one that scares the shit out of me is pandemic influenza.