Although more than 200 incidents related to legionnaires have been reported on passenger ships over the past 30 years, there is little discussion, in maritime circles, of the causes, consequences and necessary prevention measures. There is an overwhelming belief that the problem of the appearance and suppression of legionnaires on board is something that can only be taken care of by the healthcare industry alone. The truth is exactly the opposite. Responsibility for the prevention of epidemic outbreak lies solely on technical staff because the causes that lead to the colonization of the water system by legionella on passenger ships are related to problems caused by poor water circulation, water temperature, pipeline, tanks, filters and water pumps conditions. Contamination of water systems on board ships could result in severe health problems, and hence, in loss of operating hours and profit. Therefore, this book is primarily intended for technical personnel in the shipping company management and members of the Water Safety Team on board. In the example of a specific vessel, possible factors contributing to the development of legionella bacteria and concrete technological and organizational measures to limit the growth of bacteria are pointed out. The book aims at presenting the main reasons for the growth of bacteria and other parasites in ship's water systems.