Infectious pneumonia, one a common disease, shows various severity and outcomes that can be fatal. Severe pneumonia can be defined as pneumonia with acute lung injury and/or severe sepsis, which often requires ICU admission. Several clinical criteria are useful to predict ICU admission. Microbial etiology of severe community-acquired pneumonia includes Streptococcus pneumoniae, influenza virus, Legionella spp., and polymicrobial infection. Severe sepsis and septic shocks are characterized by activation of leukocytes, complements, and the coagulation system. A catastrophic cascade of inflammatory reactions may injure vascular endothelium and induce intravascular coagulation, which will eventually cause multiple organ failure. Various cytokines, including high-mobility group box 1 protein, work as mediators of both sepsis and acute lung injury. Specific virulence factors of pathogens are involved in the development of severe pneumonia. Strategy for the prevention and control against severe pneumonia should be established.