Consider the external costs of my getting the flu: I could infect a number of students through my in-class interaction with them, I could miss class meaning that many students will miss out on vital learning (and will suffer mental anguish at the prospect of having a two-hour class on international economics cancelled - the horror!) and I may infect other professors and staff at OSU. The multiplier effect of my getting the flu means I would be a super-infector spreading influenza far and wide across the OSU campus. Surely the cost to OSU of all of this is more than the $20 they are charging me for the flu shot - so why oh why don't they give it to me for free.
Students: I did shell out the $20, so you can thank me now. No worries about me cancelling those two-hour international classes at 8am!
(NB: I have been made aware that Alex Tabarrok at Marginal Revolution has a similar post - see the list of 'Economics Blogs'. OK, but I am sure I thought of it first!)