What may spread the virus maybe the airplane travel of so many people to and from the marathon, the expo, and not so much the actual race outside. I think we don't know if a person could contract the virus on an airplane to Boston, run the race, the spread it on another airplane home. Can it spread that quickly and perhaps without the runner showing symptoms?
The marathon was cancelled in 1918 and replaced by a feckless military relay to show patriotism and sell war bonds, not the "Spanish" flu of the time.
That story is in my book that of course everyone one on Let's Run has already read.
Tom