displayed irregular interstitial inflammation with moderate mononuclear infiltrate (not shown), indicative of pneu- monia, as has been observed previously in A/J mice infected with L. pneumophila for 72 h . At 24 h post-inoculation, 4.5 logs of H63T were present in the spleen, and the bacterial burden remained at that level for an additional 24 h (Fig. S4c). After 48 h, the numbers of bacteria in the spleen decreased and, by the fifth day, H63T was detectable in the spleen of only one of the five animals. In summary, H63T replicates significantly within both human macrophages and the murine lung, indicating that the strain is pathogenic, compatible with its isolation from a human patient experiencing severe illness.