“When we get a positive, we’re sure,” Alan Hay at the NIMR told New Scientist. “But when we get a negative, we aren’t.” One problem is getting a sample with virus in it. The amount of virus present during the course of bird flu in humans varies more than with human flu. And test samples are usually mucus from the nose or throat. But because H5N1 is a bird virus, it prefers the higher temperatures – and the more bird-like cell-surface molecules – of the lower lungs.
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I'm not using false negatives as evidence of a broader spread of a milder disease, rather suggesting that a number of patients who died presumably of other causes actually died of bird flu.