The bacteria Streptococcus pneumoniae colonizes the nasopharynx and can cause pneumonia. Then, it can spread to the bloodstream and cause organ damage. To understand how this pathogen adapts to different locations in the body, and also how the host responds to the microbe, researchers have measured bacterial and host gene expression at five different sites in a mouse model — the nasopharynx, lungs, blood, heart and kidneys — using three genetically different strains of S. pneumoniae.