PANDEMIC

The newly identified avian influenza variant exhibited such an exceptional capacity for rapid host‑to‑host transmission that epidemiologists warned it might be among the most dangerously ​ (a)   strains ever documented

“Younger birds, whose immune systems were not yet fully developed, proved especially ​ (a)   to the rapidly evolving avian influenza strain circulating through the region.”

The avian flu outbreak escalated so rapidly and with such complex patterns of mutation that experts described the situation as an ​ (a)   challenge to global surveillance systems

The virus’s unusual combination of genetic markers suggested the emergence of a highly adaptive ​ (a)   strain capable of exploiting traits from multiple avian lineages

Due to subtle genetic variations in their immune receptors, certain bird populations proved far more ​ (a)   to the emerging avian influenza strain than researchers had previously anticipated