There never was that much in Chicago, but there were a few big ones. Back in the day it was basically Abbott, American Hospital Supply, Baxter, and G.D. Searle and most other Chicago based companies were spin-offs from those. Baxter bought American Hospital and eventually sold off most of the pieces scattering them across the country, Monsanto bought and sold Searle, and Abbott split itself into Hospira, Abbvie, and Abbott. The rest are relative newcomers.
