This question offers a good example of the need to separate correlation from causation. Many well educated persons have jumped to the conclusion that the correlation between disease and nitrite inhaltion is proof that the latter is a health risk. Some of them have willed that to be true because they can't stand the thought of somebody else having too much fun.
Amyl was used for over a hundred years as the treatment for angina pectoris, during that time it was used by millions of patients, most of them in very poor health. It never killed any of them and has proved to be an amazingly safe drug. Many popular bad habits, like coke, meth and tobacco raise blood pressure and give your heart a hard time. Nitrite does not raise it, it lowers blood pressure, which only a problem if you already suffer from unusaly low bp. Which is a problem if you combine poppers with viagra, that will make your bp dangerously low. Not even I will mix those two, to keep your hard-on use a cockring instead.
Looking at the statistics, it seems that those who are regular popper users are at greater risk of sexualy transmitted infections, and there is a strong correlation here. But before jumping to the wrong conclusion consider all the lifestyle factors. The heavy huffers are the ones with the frantic sex lives. Look at it holisticaly. A life of late nights in the clubs and glory holes means not enough sleep, not enough good food, and usualy too much smoking and drinking. These are all factors which run down your bodies immunity.
It is interesting to look at what happened in France when there was a total ban on poppers, gay men turned to using stronger illegal drugs instead. Result, a sharp increase in crime and a sharp reduction in gay mens health. The ban was quickly and quietly reversed.
Does this stuff lower your immunity?, yes it does, most drugs do, but unlike other drugs, nitrites are very quicky eliminated from the body. The effect is too short lived to be a factor in the slow process of converting from HIV negative to positive.
There is a danger that , because this stuff makes you hornier than a dog with two cocks (or it does for me at least), you will forget all you know about safe sex and just go for it without a care. That does happen, when it does it's your own fault. Blaming the drug is just an excuse. As a grown adult you make your own decisions about the importance of precautions during sex. Good poppers make you ravenous for cock or should I say even more ravenous than usual. You can satisfy that hunger without barebacking. If you know for a fact that you are both negative, or both positive, then you can just go for it anyway, poppers or no poppers.
Nitrites increase the bloodflow on the surface of the skin, this is one of the many ways in which they add to the pleasure of sex. This increased bloodflow does mean that it becomes easier for the virus to pass from one body to the other, because it needs to get into the blood. It makes no difference for a bacterial infection like gonorrhea which attacks from the surface. In practice oral sex is still very low risk, with or without the extra bloodflow, and barebacking is still potentialy very high risk, with or without the extra bloodflow. If you don't have a need for safer sex then it makes no difference. If you do have a need to practice safer sex then the condom will protect you or your partner, with or without the extra bloodflow. But only if you actualy use it, if you forget, it's your own fault, not the poppers.
For up to date information there is a user group on Reddit, search there for "Popperpigs" no need to join to read it.
On a lighter note, here is a studboy enjoying a bottle of Jungle Jiuce Platinum which is one of the best you can buy if you can find it.
I wish I was there to share it with him.