Vibrio are bacteria that occur naturally in brackish waters such as the Delaware Bay, the Inland Bays and tributaries, especially during warm weather months.

The primary bacterial concern in Delaware's recreational waters (beaches, bays, and lakes) is Enterococcus, which is used as an indicator for various pathogens found in the intestines of warm-blooded animals. High levels of Enterococcus usually occur after heavy rainfall, which washes animal waste and runoff into the water, potentially leading to gastrointestinal illnesses, skin rashes, or ear infections. In 2026, the Delaware Department of Natural Resources and Environmental Control (DNREC) also monitors for Vibrio vulnificus, a naturally occurring "flesh-eating" bacterium found in warm, brackish waters like the Delaware Bay. While rare, Vibrio can cause serious infections if it enters an open wound. Additionally, specific "Boil Water Advisories" (such as the 2026 advisory in Frederica) are sometimes issued due to microbial contamination from a loss of water pressure, indicating an increased risk of coliform bacteria or parasites in the local drinking water system.