I just had a discussion about this with someone and am too tired from my antiviral to look up the answer. It's increasingly clear that Natural Killer Cell activity tends to be very low in many, most or all CFS patients. Here are some possible reasons: 1) NK Cell activity is low due to innate causes (e.g. heredity or maybe toxicity), and therefore allows viruses to infiltrate the body. These viruses cause the fatigue and other problems associated with CFS. This is because the viruses themselves are causing damage and/or because other parts of the immune system (specifically, interferon alpha serum) must do the work of fighting the viruses even though they're not as good at it as NK Cells are. 2) One or more "stealth viruses" have invaded the body, and NK Cells have to work especially hard to keep them under control. This wears the Nk cells out. (These stealth viruses may or may not have been identified by medical researchers....new viruses are being discovered all the time, of course.) 3) Some sort of virus attacks the NK Cells in some way, making them less effective. (This is similar to what the HIV virus does in AIdS, I think? I know little about AIDS though.....before my mind went to mush on this AV, I never thought it was relevant enough to my condition to seek out info on it.) This would be a virus that has yet to be identified (as the AIDS virus was not identified until after about two years of intensive searching). Some people are able to throw off this attack virus when it infiltrates, but others (when under significant stress and/or who have generally weaker immune systems with regard to fighting that particular bug) do not.....thus leading to chronic problems. 4) More than one of the above. 5) None of the above. This seems like a simple question, and so I can't believe I've never thought to ask it before. I've been sort of assuming one of those causes, but to ASS-U-ME is generally not a good idea. This question probably has been answered before. If it has, please supply. Otherwise, please chip in with guesses. Thanks!!!!