Although a well-functioning human intestine teems with a variety of microbial life, serious illness, long-term intravenous feeding, and multiple rounds of antibiotics wipe out much of this diversity. Inspection of 16S rRNA sequences from stool samples showed that the guts of five healthy volunteers harbored at least 40 bacterial genera. In contrast, in five of the 14 ICU patients in the study, 90 percent of the bacterial sequences were from just one taxon—typically a known pathogen, such as Enterococcus or Staphylococcus.