Scores obtained for different tissues stained by virus histochemistry using human H3N2, mallard H6N1, and gull H16N3.

The column “number tested” indicates the number of tissue cores tested for the human, mallard, and gull virus, respectively; the scores are as follows: − no attachment observed, ± attachment observed in at least one tissue core,+attachment to ≥50% of cells observed for at least one cell type in all tissue cores; the sign * indicates that very few cells were visible. Detailed scoring is provided in Tables S1, S2, and S3.