According to FAO statistics 2008, the total fisheries production was 142.29 M tons out of which 115.09 M tons for human consumption only. Besides the loss due to local pathogens, inadequate farm- management, environmental factors and poor water quality continue to be the most common causes of disease outbreaks in farmed fish and shellfish, pathogen transfer due to international trade in live aquaculture animals and their products is a major underlying reason for major epizootics. Disease outbreaks cause significant losses in aquaculture production and trade and are affecting economic development of some countries. in recent estimates, based on farm surveys in 16 Asian countries, annual losses due to disease in the region total more than US$3.0 billion. This can broadly be prevented by reducing the most important threat i.e.environmental threat on the fisheries sector. In doing so, pathogenic threat can be subsidized and a significant loss due disease and epizootic outbreak can also be minimized. This book provides an insight of various environmental insults to the aquatic organism and how they alter the metabolism of host which attract the pathogenic invasion.