Bacterial infections are prominent etiologies put the life of fish at risk with consequent negative impacts on growth, fecundity and productivity. Vibriosis, pasteurellosis, sreptococcosis, pseudomonads and aeromonads are serious infections affecting many marine aquatic animals. The pathogenesis of bacterial infections affecting aquatic animals is interrelated. Variable factors related to fish, environment and pathogen should work together in concert to define the nature of the triggered course of infection. Adverse environmental conditions are the main predisposing factors for bacterial infections affecting fish. Long exposure to stressful environmental circumstances impair fish immune defense mechanisms, ultimately, enhance invasion with opportunistic microorganisms and establishment of disease condition.