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The laboratory worker is also protected by the restrictions placed 
on the sources of DNA and the host and vectors that can be used in 
recombinant DNA experiments. The Guidelines prohibit the use of 
organisms as DNA donors if they possess intrinsic hazards greater 
than that possessed by a class 2 organism. The host organism in which 
the recombinant DNA molecules will be inserted must at least conform 
to the definition of a class 1 etiologic agent. Thus, the host organism 
containing recombinant DNA molecules must acquire the virulence, 
transmissibility, and pathogenicity that neither host nor donor organism 
inherently possess if it is to create a risk similar to that of a class 3 or 
class 4 pathogen. Such acquisition would be an event unparalleled in the 
history of microbiology. 
If host organisms containing recombinant DNA molecules were to 
acquire an unusual hazard potential, it is likely that such hazard would 
only be expressed following the ingestion of large quantities of the 
organism. This mechanism of exposure can be eliminated by the use of 
safe pipetting practice and personal hygienic measures common to our 
society. 
The safeguards required for the conduct of recombinant DNA research 
demonstrate considerable caution. They are more than adequate to 
protect the laboratory worker conducting experiments involving recom- 
binant DNA molecules. 
The experience in research with human pathogens also demonstrates 
that the potential for an accidentally infected laboratory worker to transmit 
disease to another person outside the laboratory is exceedingly small. 
It has only been possible to document eight infections that were presumably 
transmitted to other persons from laboratory -infected workers. A wife 
contracted Marburg disease 11 weeks after her husband's recovery from 
a laboratory-acquired infection (12). A wife was also reported to have 
acquired Bolivian hemorrhagic fever after visiting her husband in the 
hospital (13). Recently in England, fatal smallpox was transmitted to 
two visitors by a hospitalized vaccinated laboratory technician mildly 
ill with smallpox undiagnosed at the time of the visit (14). In four 
European surveys of laboratory-acquired infections in which 265 
infections of typhoid, paratyphoid, shigellosis, and salmonellosis were 
reported, the authors observed four secondary cases. In two cases 
typhoid fever was contracted by a patient's nurse (15); in one case 
typhoid was contracted by a patient's residential roommate and in one 
case the wife of a patient contracted cholera. An important aspect of 
each survey is that the survey questionnaire included a query as to 
whether there were secondary cases. In addition, the examination of 
laboratory-acquired infections data from the Center for Disease Control 
(CDC), National Animal Disease Center (NADC), NIH and Ft. Detrick 
reveals no case where disease was transmitted from an infected 
laboratory worker to another person. 
Laboratory workers, of course, may become infected with pathogens 
that are not under study. For example, viral hepatitis in laboratory 
workers has been reported on numerous occasions (5). Infections 
