They aren't right in the head. The term "bug-chaser" has been around since AIDS was first diagnosed. There are those who deliberately seek out getting the HIV virus so they can be part of that crowd.
I've heard of a much more sickening motive. Some have admitted that they find the ultimate sexual intimacy by taking in the integrated pathogen of the other. In other words, they like being raped down to the cellular level.
Now I need an iodine shower.