Introduction: Although Legionella genus bacteria are present in all natural water resources, their concentration levels are insufficient for human infection. When man created a pleasant environment for himself, he also created a pleasant environment for certain microorganisms. An example of this are Legionella bacteria which find perfect conditions for multiplication in the water network systems where the water temperature ranges from 25 °C to 45 °C. Purpose: The purpose of the master’s thesis was to develop a multiplication prevention plan of Legionella genus bacteria in the water network system of a larger Slovenian healthcare institution. Work methods: The master’s thesis was compiled in several stages. After examining expert and scientific literature as well as recommendations issued by several institutions, the author developed a plan for prevention of multiplication of Legionella genus bacteria in a larger healthcare institution. The plan was then put into practice and, after three months of use, duly completed and upgraded. Main part: The institution for which the plan was drafted is a healthcare facility in public use where drinking water is used directly for drinking and personal hygiene purposes. The water is used both by the healthy population (staff) as well as people with a weakened immune response (patients). The plan includes a detailed description of the work and tasks carried out by the staff. It offers a comprehensive overview of the precautionary and correctional measures needed to prevent Legionella bacteria from multiplying in the water network system. Discussion and Conclusion: The protection of people against contagious diseases consists of general and special measures for disease prevention and control. According to experts, infection with Legionella bacteria in hospitals is extremely dangerous (28 per cent mortality rate), which is why it is important to prevent patients from contacting the bacteria. The most effective way for reducing the number of bacteria and, in consequence, the infective dose, is by preventing their multiplication in the water network systems of such institutions. To achieve good results, it is essential to have multiplication prevention plans of Legionella bacteria in internal water network systems of healthcare institutions. This entails, in particular, having efficient measures in the fight against Legionella bacteria as well as documents that are clear and user friendly.