Viruses express tissue tropism and host specificity. In particular, the avian influenza strain H5N1 has rare incidences in humans due to the reduced distribution of alpha-2,3 sialic acids in the lower respiratory tract. As a consequence, when the H5N1 strain does infect a human, the disease is very severe due to the risk of pneumonia. In contrast, human influenza strains (H1N1, among others) attach to alpha-2,6 sialic acids, which are readily expressed in the upper and lower respiratory tract. Which of the following would not be true if a mutation were to allow the H5N1 strain to attach to alpha-2,6 sialic acids?A. The avian influenza strain could be transmitted easily from human to human.
B. An epidemic would likely ensue due to the lack of immunity to the new avian influenza strain.
C. The avian influenza strain could be transmitted directly from birds to humans.
D. The avian influenza strain would only infect the lower respiratory tract of humans.

If there is mutation which allows the H5N1 strain to bind to alpha-2,6 sialic acids, it will lead to a transmission from human to human, immunity for the disease will be lost leading to susceptibility, and the avian influenza will be able to be transferred to human.

The avian influenza affects the entire respiratory tract and not just the lower part hence option D is wrong and is thus the answer.