In older U.S. cities, heavy rainfall doesn’t just flood basements. It can also send a wave of dangerous pathogens into municipal water sources. Lakes, rivers, streams and even reservoirs may see an uptick in sewage-borne bacteria and viruses after heavy rains, infecting boaters and swimmers and overwhelming the ability of treatment plants to purify drinking water, according to Jyotsna Jagai, MPH05, N09.