The feared bacteria is responsible for legionellosis, an infectious disease that can lead to pneumonia. In order to infect us, this pathogen has developed a complex method enabling it to camouflage itself and go unnoticed in our cells, thus avoiding these acting against the infectious bacteria. Research led by the Basque biosciences research centre, CIC bioGUNE, in which teams from the National Institute of Health (NIH) of the USA and the National Supercomputation Centre in Barcelona (BSC) have also participated, has described for the first time a mechanism that aids this bacteria to camouflage itself in human cells. The research, recently published in the prestigious PLOS Pathogens, resolved the structure of the SidD protein of the , involved in the interference of cell processes during infection. , which lives in stagnant water, gains access to our organism through the respiratory tract, when we inhale contaminated microscopic drops of water. Once there, the cells of the immune system phagocytise it (that is, they ‘swallow’ it up), but are not able to destroy it. This is due to the fact that the bacteria manages to manipulate the host cell to go unnoticed inside it and multiply itself without being destroyed. The strategy of the bacteria involves releasing about 300 proteins into the cell, which in turn act on the host proteins in order to avoid being recognized as an infectious agent and thus go unnoticed during the time necessary to multiply itself. One of these proteins, concretely the Legionella SidD protein, regulates a chemical modification involved in the process of intracellular camouflage. It is precisely the function of this protein which the research by CIC bioGUNE, the NIH and the BSC has described. Once the Legionella has managed to multiply itself, the SidD protein unblocks cellular processes which favour the advance of the infection. “ is an organism which, during millions of years of evolution, has learnt to manipulate our proteins to its own benefit in order to thus favour the infection”, explained CIC bioGUNE researcher, Aitor Hierro. “Knowing how it does this”, he added, “can help us to manipulate our proteins to our own benefit”. The discovery of the mechanism enabling the bacteria to survive and develop within our cells could give rise to the discovery of new findings. According to Mr. Hierro, “this knowledge not only reveals new targets that can be used for designing inhibitors, but also shows us molecular mechanisms that can be readapted and used, for example, in the selective transport of molecules for therapeutic use”. Legionellosis was thus named in 1976 when an epidemic outbreak of pneumonia amongst participants in a convention of the American Legion in Philadelphia (USA) was described for the first time. This pathology was caused by the bacteria, which lives in stagnant water and transmitted by air when we inhale microdrops of contaminated water in suspension. The most important foci of infection are located in water systems in refrigeration towers in large buildings, such as hotels, hospitals or spas. Contamination at one of these installations can cause an outbreak of legionellosis amongst those in the vicinity. The infection by this pathogen can cause diseases of a very varied prognosis, the most well-known being legionnaires' disease, a severe respiratory infection which may involve pneumonia and which has a high mortality rate unless a suitable antibiotic-based treatment is employed. The other disorder caused is Pontiac fever, a much less serious illness which is accompanied by high temperatures, which last for short periods, and which cures itself. Structural Basis for Rab1 De--‐AMPylation by the Legionella pneumophila Effector SidD. Yang Chen, Igor Tascón, M. Ramona Neunuebel, Chiara Pallara, Jacqueline Brady, Lisa N.Kinch Juan Fernández-Recio, Adriana L. Rojas, Matthias P. Machner, Aitor Hierro.