Heroin is not heated to avoid contamination but rather to treat soluble or poorly soluble adulterants and dilutents. IF you have very impure heroin , chances are it will require a decent amount of hear ("decent" being all of 5 seconds). Some forms like Tar (an Acetate)will take alot longer, or rather short but repeated heating and agitation.
That said, contamination does happen, espcially with the afore mentioned Tar, Botulism anybody? Microns, as Fast correctly noted will not catch viruses. They are 3 times as effective as say cotton, and are great for cleaning up tablets (getting out all the talc, starch, and what not).
Heating the 3 or 4 seconds most users apply is not going to hurt it so why not err on the side of safety?Even in Cambodia when I was IMing as pure as it gets I was heating.All it takes is one misstep to ruin your life. 3 or 4 seconds is not going to cost you that much of your experience so better to just do it.
CaptHeroin: HIV exists outside the body, absolutely, since you can gain it fromn a utensil this should be self evident. However, it does not survive long and must be in the correct medium to survive at all.
HCV (AKA Hep-C), which I have by the way, does not survive indefinitely but does survive much more heartily than HIV IF given the proper medium. All in the medium.