Smoking and lower preexisting immunoglobulin G levels were strongly associated with M. pneumoniae respiratory infection. These findings emphasize the importance of immunity and cessation of smoking for the prevention of disease. The high attack rate emphasizes the extent of infection transmission among healthy persons living in close contact.

Shigella sonnei infection is an inconvenient and possibly serious health threat to travelers, expatriates, and soldiers who enter less-developed countries. For example, S. sonnei was responsible for 54% of all Shigella infections in a survey of almost 2000 clinical isolates collected over a 13-year period from Finnish travelers (calculated from data in reference 9). Of travelers suffering from shigellosis while they were visiting Africa, 43% were infected with S. sonnei, and 51% of the travelers with shigellosis in the Far East were infected with this species. For Shigella-positive Finnish travelers in Europe, South and Central America, the Soviet Union, and the Middle East the percentages of infection with S. sonnei isolates were somewhat higher, ranging from 60% to 67%. These and other data suggest that a vaccine protecting against S. sonnei would eliminate a majority of the shigellosis cases experienced by travelers, expatriates, and soldiers. For example, such a vaccine would have protected troops from almost 90% of shigellosis during Operation Desert Shield (10).The predominance of S. sonnei as a cause of diarrhea in travelers is sometimes in stark contrast to the occurrence of this species among indigenous peoples. For example, S. sonnei was isolated from only 3% of native patients suffering from shigellosis in Sub-Saharan Africa, 5% of native patients in South Asia, and 15% of native patients in East Asia and the Pacific. This species did not predominate in native patients with shigellosis even in the Middle East (29% of isolates) or in Latin America (31% of isolates) (16). It is well known that the general level of environmental and personal hygiene affects the proportions of shigellosis that are attributable to S. sonnei and to Shigella flexneri. Improved hygiene reduces the ratio of S. flexneri (serogroup B) to S. sonnei (serogroup D) (B:D ratio) (2). Importantly, the B:D ratio can vary greatly within a relatively small geographical area. In southern Israel, for example, S. sonnei causes more than 70% of the shigellosis in the urban Jewish population of Beer-Sheva, while S. flexneri causes almost 70% of the shigellosis in the Muslim Bedouins living in the adjacent Negev desert towns and settlements (6). Since travelers from industrialized countries tend to lodge and dine in comparatively well-developed urban environments, they are exposed to S. sonnei more often than would be predicted from the overall B:D ratio for a less-developed country.

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Our study demonstrates the possible ramifications of the combination of a virulent and highly infective S. pyogenes strain and poor living conditions, and it emphasizes the importance of early intervention in such conditions.