virulence pneumoniae streptococcus capsule pneumococcal membrane nieto polysaccharides structures cytoplasm layer bacterium
pathogen host pathogenicity bacterial interactions interaction microbiology
upec virulence factors schematic bacterium fig urinary tract schembri bacterial
antigenic virulence drift factors shift viruses bacterial viral pathogens influenza host virus variation hemagglutinin strains genetic microbiology mechanisms summary flu