On Dec. 4, 2011, some 44,000 runners participated in the annual Rock ‘n’ Roll marathon in Las Vegas. About 500 got sick with sapovirus.

A health district investigation determined that people were probably exposed to the virus the morning before the race — maybe at a runners’ expo where race numbers were distributed, although water distribution during the race had been suspected.

This year, organizers are promising several changes, including strict sanitation rules and a contractor supplying water to runners.