Koi herpesvirus disease (KHVD) is a highly contagious and virulent disease, and therefore, is a significant threat for carp and koi farms and for freshwater ecosystems. The pathogen of the disease is Cyprinid herpesvirus 3 (CyHV-3), also known as Koi hepesvirus or KHV. Here, I talk about the relation between the dynamics of the CyHV-3 and the environmental factors. At first, based on the results of a nationwide river survey and 5-years monitoring in a lake (Lake Biwa, Japan) and a river (Yura River, Japan), the ubiquitous presence of the virus is clarified. This result indicated that virus invasion does not consistently cause an outbreak and that some environmental factors may be involved in its occurrence. Next, the possible stories of KHVD spread, which start from the environmental alteration by humans, were investigated by the combination of field observations and laboratory experiments. The results showed that the lake-shore alteration could enlarge the size of outbreak via increased temperature stress to the host species and overcrowding of carp at limited breeding area. From these studies, we can conclude that KHVD spread is partly caused by environmental alteration by humans.