Was just reading this thread, and I wanted to point out that a couple people said cotton fever or dirty hit syndrom comes from an immune reaction to bacteria. However thats not completely true. It is an immune response but it's not directly from bacteria. It is from an endotoxin produced by a bacteria that commonly grows on cotton.
The way your body identifies attackers is through the use of antibodies. Antibodies 'tag' attackers so lymphocytes can come through and consume them, or envelope them to neutralize said attackers. The antibodies however only interact with smaller protein molecules located on the outside of bacteria though, rather than the whole bacteria itself, so my guess is that the endotoxin this bacteria produces is probably similar to a protein used to identify many other bacteria and although it's harmless, your body freaks out thinking it's being invaded by a bunch of bacteria, when really it is just the small endotoxin protein.
Also still just guessing here, but this protein is more than likely small enough to float on through a micron filter unmolested, while bacteria themselves cannot. Hence why some still get the reaction even when micron filtering.
Lastly, the reason I was seraching for dirty hit information is because after an injection I got immense joint pain and a headache... I thought i was dieing. This came on immediately after injecting, maybe like 30 seconds peaking after 5 mins. It felt like every joint i had was in a vice grip but especially my lower back and neck. After another hit at another time I determined it was the Q-tips I was using... "Swissper's" brand. I suspect something that they use to bind the cotton to the plastic tip causes the reaction. So anyway, not to hijack the thread but I thought I would post what info I had.