Why does the author give human characteristics to the parasites and their animal hosts throughout the text in "Your Average, Everyday Zombie"?

I haven't read the text, however this is a common technique used in literature in order to criticise society. In many texts with this intent, the author will associate humans with bacteria, animals or parasites to create an 'infected body politic' with a semantic field of sickness and disease. The overall motivation for this is to mock society or to highlight a particular flaw with humans.