Foer begins with a discussion of the flu, or influenza, which is a type of virus that has caused pandemics across the globe. Discussing the 1918 Spanish flu, as well as more minor outbreaks in the 20th and early 21st centuries, Foer links each virus to birds. He describes how birds host a variety of viruses, only some of which are capable of infecting humans. Zoonotic illnesses are those that manage to cross from animals to humans, and Foer notes that factory farming increases the chances of these viruses becoming zoonotic, human infections.

Outlining a poultry farm, Foer describes how 33,000 birds will be kept in a shed, each with less than a square foot of space. The birds usually cannot function, and most catch diseases like E. coli. Despite the deformities, producers are able to inject and manipulate the meat to obscure such conditions. Workers also suffer physical consequences, as they are expected to handle significant numbers of living and dead birds, exposing them to injury and disease.

Shifting the focus back to the flu, Foer notes that most chickens that are factory farmed are shipped to market infected with one or more harmful bacteria, resulting in 76 million cases of foodborne illness in the US each year.