Mutations are the ultimate source of genetic variation, but not all the variants found are the result of structural or point mutations. Bacteria have developed specific strategies for incorporating and exchanging genes and plasmid segments of external sources. The processes of non-vertical transmission allow microorganisms to acquire new genes or variants which provide new adaptations such as resistance to environmental processes and to antibiotics both in a short and long term. "The ST578 type of L. pneumophila has caused outbreaks during more than 10 years in Alcoy, and in this study, based on massive sequencing analysis we managed to see that some of the outbreaks have not been caused by a single biological source. The results show that the evolution of this population of ST578 has been incited by the introduction of a variant with the same profile, from external sources of the water distribution network of the city. "All this has occurred despite the remarkable success of prevention and control measures carried out by the health authorities that contributed to the absence of outbreaks between 2006 and 2009," explains Sánchez-Busó.