Well, no one has done blood tests of every single human on the planet if that's what you're getting at, but in the limited communitywide blood testing done so far in areas with H5N1 outbreaks like in Hong Kong and Vietnam, there is no evidence of widespread but unnoticed H5N1 infection. Basically, if the H5N1 virus can get a toehold in your body, you're going to get extremely ill.

Roughly half of the laboratory-confirmed cases of H5N1 infection have ended in death. By any medical standard, that's an extremely lethal disease. Very few infectious diseases kill that many people even if you limit your denominator to just those sick enough to seek medical care. For comparison, Spanish Flu "only" killed a couple percent of those who fell ill.

Genetic reassortment between H5N1 and a regular human influenza virus could theoretically give it the ability to easily pass from human-to-human and result in a full-blown pandemic strain literally overnight. That's the nightmare scenario that medical experts worry the most about because there won't be any warning signs before there's an explosion of new cases. H5N1 does not have to slowly mutate and evolve human-to-human transmissiblity on its own over time, although it could do that, too.