I also noticed they throw in the words diversity and unity quite a bit in this textbook. They like those words. Biological 'diversity' was used much before it because a 'common' term. Dummy.
And of course AIDS/HIV has to be mentioned. Again, it was mentioned because the HIV virus mutates and that is the key to why it is so drug resistant.
"...HIV which causes acquired immune deficiency syndrome (AIDS) is a retrovirus.... an RNA genome with the standard retroviral genes along with several other unusual genes......The reverse transcriptase of HIV is even more error prone than other known reverse transcriptases - ten times more so - resulting in high mutation rates in this virus... One or more errors are made every time the genome is replicated."
This rapid mutation rate will lead to quick drug resistance and even changes in host specificity. It also makes it easier to defeat the hosts immune system.