Initially, we were unsure whether the outbreak strain was a pneumococcus. Typical pneumococci have a polysaccharide capsule, which is considered a key virulence factor. Among 14,643 isolates of pneumococci causing invasive disease from 1998 through 2001, 99.6 percent had an identifiable capsular serotype (unpublished data). DNA–DNA reassociation is considered the definitive test for the evaluation of a species.22 Preliminary results of such testing indicate that this unencapsulated strain is S. pneumoniae (unpublished data). Other evidence collected to date, including susceptibility of the isolate to optochin, solubility in bile, reaction to a DNA probe, results of multilocus sequence typing, and sequencing of 16S rRNA, also suggests that this strain is a pneumococcus. Further studies are needed to explain how, in contrast to typical pneumococci, certain unencapsulated strains are capable of causing large outbreaks and why they cause conjunctivitis rather than other pneumococcal syndromes. The high attack rate suggests that few students had preexisting immunity to this strain. The absence of a capsule may limit this strain's ability to cause more serious infections.