The need for more and more food fish to feed a growing population gave rise to aquaculture. Today, aquaculture yields have increased, with more harvests coming from farms in Asia. Aquaculture has three phases – the (1) hatchery, (2) nursery and (3) grow-out phases. Most hatchery operators use tanks to hold the young organisms that they grow. In the nursery and grow-out phases, tanks, ponds, and floating cages hold the farmed animals until harvest time. In aquaculture, any one of three production systems may be used in growing the chosen species. These are the extensive, semi-intensive, or intensive production system. Choice of a system depends on the desired density of animals to be farmed in a given area. In the intensive system, fish are farmed in high stocking density. High stocking density results in exposure of the animals to stress that often leads to disease. Disease outbreaks, in turn, cause production losses due to lower harvests or aquatic products of poor quality. Disease is defined as any abnormality in structure or function displayed by living organisms through a specific or non-specific sign (symptom). Infectious organisms, wrong management practices and environmental problems can cause disease in farmed aquatic animals. Tissue or organ damage, reduced growth rate, or death may indicate disease in fish. The consequence of disease includes rejection of aquaculture products and the loss of productivity. Persistent disease occurrence might cause the collapse of aquaculture ventures and threaten the sustainability of the industry as a whole. Because of their harmful effects, disease and environmental problems have gained worldwide attention. Although economic losses due to diseases in aquaculture are difficult to measure, data gathered from the export of various aquatic commodities may serve as indicators for losses or gains in production. For example, China’s export figure for farmed shrimp in 1992 was 140,000 metric tons. In 1993, shrimp export went down to only 30,000 metric tons. Viral disease caused the 79% reduction. Translated to export earnings, the country lost about a billion dollars from shrimp alone. An ADB/NACA (1991) estimate of losses in aquaculture due to disease was about US$1.36 billion in 15 Asian countries in 1990. The situation is even worse at present with viral disease plaguing shrimp culture facilities worldwide.