Influenza (the flu) is responsible for approximately 250,000 to 500,000 deaths annually, but periodically its toll has been much higher. For example, the 1918 flu pandemic killed approximately 30 million people worldwide and is considered the worst spread of a deadly illness in recorded history. With highly virulent flu strains emerging periodically, it is little wonder that the scientific community is actively studying influenza biology. In 2007, the National Institute of Allergy and Infectious Diseases completed sequencing of 2035 human and avian influenza virus strains. Influenza strains undergo recombination as described in this chapter, and they have a high mutation rate owing to the error-prone replication of their genome (which consists of RNA rather than DNA). In addition, they are capable of chromosome reassortment in which various combinations of their eight chromosomes (or portions thereof) can be packaged into progeny viruses when two or more strains infect the same cell. The end result is that we can make vaccines, but they must change annually, and even then, we can only guess at what specific viral strains will be prevalent in any given year. Based on the above information, consider the following question.
Of what evolutionary value to influenza viruses are high mutation and recombination rates coupled with chromosome reassortment?
a. These processes allow influenza virus strains to reproduce in sufficient quantities such that the immune system cannot immediately confront all of the infected cells.
b. Due to these processes, influenza viruses infect the host cells and mutate to be similar to the components of the cells, and evade the immune system.
c. These processes prohibit genetic diversity enabling viral survival as host defenses adapt.
d. These processes provide genetic diversity enabling viral survival as host defenses adapt.