The authors point out the remarkable importance that pneumonia has today among infectious diseases, and survey the main risk factors and etiological agents both of the forms acquired in the community and in the hospital, also considering the data from the international literature. The authors stress the high incidence of gram-positive microorganisms, among which Streptococcus pneumoniae is the most widespread in the forms acquired in the community, and the absolute prevalence of the gram-negatives in the nosocomial forms and, among the gram-positives, the pre-eminence of Staphylococcus aureus; these are "difficult" bacterial species, whose prevalence has been determined mainly by induced selective pressure through the inadequate use of antibiotics. The authors also point out the principal factors that favor the onset of pneumonia; they are connected to the host and to the environment; their combination with the infectious agent causes the infective event.