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(ICDDR,B, 1998). It is estimated that 60% of the world's population lives in coastal zones, the area within a few kilometers of the shoreline. Disease incidence is increasing worldwide, promoted by both natural phenomena such as El Niño and human activities, including sewage disposal. Ancient diseases like cholera still cause epidemics (Plate V) while new agents of disease (e.g., hepatitis E and Vibrio vulnificus) continue to be discovered. Although the pathogenesis of diseases such as cholera and dysentery is well understood, the cause of outbreaks of these diseases is unresolved. Experts still debate how cholera epidemics arise despite a detailed understanding of the genetics, chemical structure, and mode of action of the cholera toxin. Cholera is known to reside in human hosts and spread by the oral fecal route, however, epidemics may be seeded by vibrios that reside in estuaries and other saline waters and infect people through contaminated drinking water and seafood. The molecular structure of recently discovered disease agents has confirmed their existence on Earth for thousands of years. The apparent emergence of these pathogens could be the result of anthropogenic influences or may reflect more sensitive modern detection technologies. Whatever the cause, it is clear that infectious diseases, including waterborne diseases conveyed by the ocean, still plague humankind.

The Agents of Waterborne Disease

The principal agents of diseases that derive from seawater and seafood are viruses and bacteria. Most disease appears to result from ingestion of contaminated seafood (IOM, 1991), although accidental ingestion of seawater (e.g., recreational exposure or contamination of potable water with seawater) is another important route of infection. Some agents enter the human body through wounds (e.g., puncture wounds from sea urchins and cuts from fishing gear) others—Leptospira—although rarely contracted from seawater, enter through broken skin, or penetrate mucous membranes, and a few—avian schistosomes—penetrate unbroken skin.

Viruses are obligate parasites and require living cells for reproduction. Certain viruses, however, can survive in seawater for long periods of time (e.g., hepatitis A, poliovirus) and are concentrated by marine bivalve mollusks, such as oysters and clams. Human waterborne viral infections result from contamination of seawater or seafood by sewage.

Unlike viruses, most of the pathogenic bacteria do not require human hosts for replication. Indeed, some are naturally occurring in estuaries and the coastal ocean, and can grow on, or within, many animals, ranging from zooplankton to fish, and on marine plants, ranging from phytoplankton to macrophytes. In general, bacteria that cause seawater- and seafood-borne diseases in humans come from two different sources. The foreign or allochthonous bacteria come from humans and other animals by means of fecal contamination: sewage outfalls, septic tanks, and land surface runoff. One exception to this generalization for