I suspect there are a couple of reasons for this impression. 1. cfs sufferers mix less with others. 2. You are already suffering from a viral infection which as stated earlier has upregulated much of your immune system. As with most viral infections we are suffering the effects of our immune system not the virus itself. So we may get infected by the cold virus but the symptoms may be attenuated. After all cfs is far worse than a cold. Of course, because we are all so different this may not apply to some people and they may experience the opposite. I think if you get hit with an active cold virus to which you have no immunity ie. no antibodies, you will get a cold. 3. Maybe there are different forms of cfs. From my reading some people with cfs (and remember every case of cfs is diagnosed on the basis of guesswork) have significantly upregulated cytokines whereas other don't so on this basis you would expect some differences in how cfs-diagnosed people respond to a new cold or flu virus.