According to the NYTimes, the most common and potentially life-threatening marathon-associated ailments are “too hot,” “too cold,” “too dry,” “too wet” and “wobble and fall down”. These translate into heat stroke, hypothermia, dehydration, low blood sodium and collapse, respectively.

The medical professionals at this year's NYC marathon had ice-water dunk tanks to treat runners who developed high fevers. Handheld i-Stat machines enabled them to analyze the chemistry of the runners’ blood, then dispense the right amounts of intravenous fluids, salts and sugar.

Understanding the Heart Hazards of Marathon Running - Video - TIME.com.

References:
Doctor Prepared for the Worst at Marathon. NYTimes.
Faces at the Finish - Interactive Feature - NYTimes.
Running With the Elites - Slide Show - NYTimes.