The virus that affects amphibians is an iridovirus that is tremendously resistant and is transmitted easily. It appears preferentially in degraded environments where the amphibians have to concentrate themselves in high numbers to reproduce, though also occurs in well conserved areas. The symptoms of the resulting disease are complicated and while many times the individuals infected die without external symptoms other times they produce local haemorrhages, skin ulcerations, and in general acute necrosis in internal organs. Also secondary bacterial infections are often produced in animals weakened by the virus, producing the symptoms known as "red-leg" (haemorrhages and severe inflammations over all the limbs). Some species of fish may also be affected by the virus and be responsible for its introduction into the environment. Fish may act as efficient reservoirs or host species, in which the virus can easily amplify. New species of this virus are being described with much speed and its implication in natural episodes of mass mortalities of amphibians is known from many places all over the world. However what appears to be a far greater threat to amphibians is the action of a certain fungus which appears to be related more directly to the global decline of amphibians.