Because 26.2 miles briefly rebalances immunity: a meta-analysis of 7 studies found an 18% higher rate of upper respiratory infections after marathons (OR 1.18), and studies of non-elite runners show the race dips salivary immune protection - the mouth and throat's antibody barrier.
Exposure does the rest: start-corral crowding, travel, taper-week sleep disruption. The dip is real but small and temporary - days, not weeks. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Why do I get sick after running a marathon?
Because 26.2 miles temporarily rebalances your immune system, and the window afterward is real. A meta-analysis of 7 studies found an 18% higher incidence of upper respiratory infections after marathons (OR 1.18, consistent across studies) once the authors corrected for uneven follow-up windows - the old "open window" theory survives in a modest, measurable form. The mechanism, from studies of non-elite marathoners: the race itself produces a transient dip in mucosal defense - salivary immune protection (the mouth and throat's antibody barrier) drops post-race - layered on top of exposure: thousands of people breathing the same start-corral air, travel, taper-week sleep disruption, and accumulated training stress. The practical frame: your immune system is not "weak" after a marathon - it is briefly redistributed, and the infection risk bump is real but small. Race-week defense: prioritize sleep in taper week (the recovery lever with the best immune evidence), hydrate and fuel through the race, wash hands through expo and travel days, and treat the first 3 to 5 days post-race as a genuine recovery window - real rest, not just lighter mileage.