Disease outbreaks have driven declines in some threatened species on land, including amphibians, bats, and birds. In the oceans, infectious diseases can also drive declines and are a potential agent of community change and threat to marine biodiversity. Recent disease-driven mass mortalities have affected a range of marine biota. This chapter outlines four case histories of disease outbreaks that affect marine communities and, in multiple cases, contribute to endangerment or listing as endangered species. The case histories include disease impacts to foundation species of corals and seagrasses, herbivores like abalone, and keystone predators like sea stars. Multi-host diseases in all these cases have contributed to extreme population declines. Infectious diseases pose the greatest threat to marine communities when they reduce foundation species like corals and seagrasses. In turn, disruption of marine communities may impact the services they provide to humans and other organisms through pathogen biofiltration. The authors suggest that marine disease impacts may be slowed by protecting the natural services provided by intact ecosystems.