A deadly bacteria is creeping up the Atlantic Coast. How worried should you be?

A small number of Vibrio species can sicken and even kill. In worst-case scenarios, a person who has been exposed to the most dangerous of them — by swimming in brackish water with an open wound or ingesting a piece of raw shellfish that is contaminated with the tasteless and odorless toxin — may find themselves with only hours before the flesh on one or more extremities starts to bruise, swell, and decay. Without the quick aid of powerful antibiotics, septic shock can set in and lead to death. Anyone can get infected, though it is much more likely in people who have liver disease or are immunocompromised, elderly, or diabetic.

Climate change is making the world’s oceans, which have absorbed more than 90 percent of the excess heat trapped by greenhouse gas emissions, more hospitable to Vibrio. Research shows that temperature and salinity are the largest predictors of how widespread Vibrio bacteria are. As water temperatures rise, so does the concentration of Vibrio in seawater — boosting the risk of infection for beachgoers and shellfish consumers. The bacteria start getting active in water temperatures above 60 degrees Fahrenheit and multiply rapidly as coastal waters warm throughout the summer.

In recent years, scientists have documented Vibrio expanding into places that were once too cold to support the bacteria, pushing as far north along the U.S. East Coast as Maine and appearing with more prevalence in temperate seas around the world.

Vibriosis infections in general are the leading cause of shellfish-related illness in the U.S. They have increased “more than any other illness caused by a pathogen in the U.S. food supply” since the Centers for Disease Control and Prevention, or CDC, started keeping tabs on such illnesses in 1996, according to a 2019 analysis by the International Association for Food Protection. The report attributed the precipitous rise to a “perfect storm” of factors that include climate change, food handling practices, expanding globalization, a patchwork of regulatory oversight, and improved diagnosis.