The table summarizes the many newly recognized agents causing lower respiratory tract infections: gram + cocci, Streptococcus agalactiae, enterococci; gram-rods, acinetobacter, Aeromonas hydrophila, Eikenella corrodens; Mycobacterium tuberculosis resistant to multiple drugs, intracellular organisms and viruses. Some details are given in four chapters: Pneumococci resistant to penicillin G which must be detected and impose new therapeutic strategies; Chlamydia pneumoniae with its difficulties for the diagnosis and the absence of standardized therapeutic trials; Streptococcus pyogenes with a recrudescence of severe and invasive infections, TSS and virulence factors; the pulmonary Hantavirus syndrome with 105 patients recognized (mortality 52%) in the USA and Canada, the homeland of the deermouse (Peromiscus maniculatus).