In 1999, we discovered that juvenile Caribbean spiny lobsters (Panulirus argus) in the Florida Keys were infected with PaV1 (Panulirus argus virus 1), the first naturally occurring pathogenic virus reported from lobsters. The virus profoundly affects their biology and ecology. PaV1 is probably wide-spread in the Caribbean with confirmed infections from the United States (Florida), St. Croix, Mexico, and Belize; and anecdotal reports from the Bahamas and Cuba. Mean prevalence in the Florida Keys has been stable since 1999 (5 - 8%), but has risen from 2.7% to 10.9% in Mexico (Puerto Morelos), the only other country where it has been studied extensively. The disease is most prevalent (> 15%) in the smallest juveniles lobsters (< 20 mm carapace length) and declines in prevalence among larger juveniles and adults. Although adults do not present the characteristic signs of this disease, they can harbor the virus with PCR-confirmed infections of adult, fishery-caught lobsters of 11 and 50% in Florida and Belize, respectively. The virus is lethal; infected lobsters die over one to several months with more rapid mortality for small juveniles. Infected lobsters become increasingly sedentary and cease feeding, often dying of metabolic exhaustion. Routes of viral transmission include ingestion, contact, and for early benthic juveniles, transmission through seawater over a few meters. Recent studies show that PaV1 is not viable in seawater for more than a few days, but larvae and postlarvae can be carriers over potentially long distances. Lobster ecology is dramatically altered during the course of infection. Prior to infectiousness, healthy lobsters avoid diseased lobsters, presumably reducing their risk of infection and resulting in infected juvenile lobsters dwelling alone rather than in groups. Avoidance results in increased shelter competition between healthy and diseased lobsters, with greater predation on the increasingly lethargic and solitary infected lobsters. Little is known about the prevalence or impact of PaV1 outside of Mexico and the United States, but the disease threatens fisheries throughout the pan-Caribbean range of P. argus. Marine diseases are emerging at an accelerated rate and the tools and knowledge that we develop through the study of diseases such as PaV1 will be invaluable in addressing future epizootics.