Long read, but essential bits are bolded (I still recommend you read all of it!)
Based on what I've read, this "fetish" seems to have two categories: HIV/AIDS and the temporary or less deadly infections. These two sub-fetishes more than likely have two completely different motivations. It's more questionable whether the former is even a real fetish.
Excerpt from a 78 member group on Experience Project for these people:
"I am a dirty man in London UK, loves Dirty NSA bareback with anyone, dirtier the better,highly turned on by risky unsafe sex with strangers or street prostitutes who could have STDs, and * in others leavings, I love * and eating a dirty, unclean, smelly *, love to and eat a * full of other guy's , or oozing period goo, and sucking dirty, dripping anus.I love the taste and juices. I like really dirty, * anyone *, who have risky unprotected sex with strangers,women with herpes are sexy, women commonly referred to as "slags" or "**" are very desirable. I wear a strap and to use an erection enhancer called muse,which works great,this will keep my hard and throbbing for around an hours raw, dirty, creamy *. I have had STDs treated and had sex with women knowing they have an infection.If I pick up a street girl, I like ones who look dirty, and if I finger them and find they have some creamy discharge I want them all the more, I really like black and asian prostitutes and the thought of one giving me herpes is a huge turn on."
This is the most explicit account I could find from the whole group. Here, a straight male is more focused on the idea of it being just another way of taking "dirty" further than most people would, kind of like a scat fetish. There are already many parts of sexuality (both mainstream and in fetishism) that revel in making it as "gross" as possible. This seems to be just an extreme (albeit rare) version of that. The nod towards black and asian prostitutes could be a subconscious hint towards getting the lowest of the low (since those races are often seen as lower class).
Another thing to point out from the experience project is that no straight males on the site were as fixated on HIV/AIDS. I'm sure there are plenty of straight people out there that do sexualize it, but it seems to be something more likely to happen among gay men. I'm using these as comparison points.
Video Notes: AIDS activist. He knew at least 30 men that had a "fetish" and caught HIV on purpose. They all ended up deeply regretting it. Claims the mentality of many people is a frustration of not wanting to wait to catch HIV and taking control(?) in an odd way. They want to hurry up and go out and get it so they can "relax". Having this type of defense mechanism be more common among gay men would make much more sense, since the risk is already naturally higher for them.
video for whoever wants to see
There's an AMA on Reddit (I am a bugchaser, HIV turns me on) that seems to contradict this, or at least be one little exception. He claims he wanted HIV when he caught it from an ex-boyfriend, and it still turns him on. Even with the fetish, he keeps up with health. He doesn't actively try to catch any other STD, but still fantasizes about the idea.
Interesting comment from the vid I wanted to include:"I brought up the subject of bug chasing in my Psychopathology class and everyone in the class looked at me like I was stupid. The professor didn't know anything about it either. Mind you, this is a doctoral program in psychology. These are future psychologists who denied the existence of bug chasing, and were ignorant to the reality of it. Very sad. :("
It'd be cool to see more stuff written about bug chasing in the future from an expert's point of view.
I'm still not sure on whether catching HIV/AIDS is really a fetish or not. I have no doubt that the pettier bug chasers are a legit fetish, even though it is definitely one of the more depraved ones. I still don't know for sure how much of the HIV thing is a defense mechanism and how much is just another example of really fucked up sexuality. Probably both?