virulent strain, Legionella pneumophila serogroup 1, can cause infections when inhaled as an aerosol or aspirated. Individuals who are immunocompromised and those with pre-existing chronic respiratory and cardiac disease are more susceptible. Legionnaires' has been infrequently associated with cruise ships. Enterotoxigenic Escherichia coli (ETEC) Enterotoxigenic Escherichia coli is a form of E. coli and the leading bacterial cause of diarrhea in the developing world, as well as the most common cause of diarrhea for travelers to those areas. Since 2008 there has been at least one reported incident each year of E. coli on international cruise ships reported to the Vessel Sanitation Program of the Centers for Disease Control, though there were none in 2015. Causes of E. coli infection include the consumption of contaminated food or water contaminated by human waste. COVID-19 News outlets reported several cases and suspected cases of Coronavirus disease 2019 associated with cruise ships in early 2020. Authorities variously turned away ships or quarantined them; cruise operators cancelled some port visits and ultimately suspended global cruise operations. People aboard cruise ships played a role in spreading the disease in some countries. Environmental impact Cruise ships generate a number of waste streams that can result in discharges to the marine environment, including sewage, graywater, hazardous wastes, oily bilge water, ballast water, and solid waste. They also emit air pollutants to the air and water. These wastes, if not properly treated and disposed of, can be a significant source of pathogens, nutrients, and toxic substances with the potential to threaten human health and damage aquatic life. Most cruise ships run (primarily) on heavy fuel oil (HFO), or "bunker fuel", which, because of its high sulphur content, results in sulphur dioxide emissions worse than those of equivalent road traffic. The international MARPOL IV-14 agreement for Sulphur Emission Control Areas requires that cruise ships must use fuel containing no more than 0.10% sulphur or make use of exhaust gas scrubbers to reduce sulfur oxide emissions to no worse than an engine running on <0.1% sulfur fuel. Cruise ships may use 60 percent of the fuel energy for propulsion, and 40 percent for hotel functions, but loads and distribution depend highly on conditions. It has been claimed that air pollution from maritime transport, including cruise ships, is responsible for 50 000 deaths per year in Europe. Some cruise lines, such as Cunard, are taking steps to reduce environmental impact by