Two factors are at play here, increasing waste from an ever expanding human population; all that waste enters the worlds waters as sewage, sometimes untreated. We measure viral loads in population of people from wastewater sampling; if you can get it they can get it. Worse, industrial pollution lowers the total amount of trac biochemicals marine mammals need to manage infection. An easy example is selenium, vital as a stylist in production of the primary lipid antioxidant (GPx3) this scarce trace element is not biologically available when fossil fuel pollution is present, instead it sequesters the selenium in a form that can not be absorbed thus removing it from the biosphere; this happened in Congo when the rail was being installed there and spread HIV all up and down the country it he 1930s. Increase the viral load and decrease selenium has only one possible outcome and it's not good.
