e397the 1980 s and during the period of epidemic rise (1990)(1991)(1992)(1993)(1994)(1995) such strains were either registered at a very insignificant level (2-4%) or not registered at all. In the period of low diphtheria incidence following an epidemic rise (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004), the concentration of nontoxigenic tox gene carrying strains considerably increased (up to 17%).Conclusion: Polymorphism of tox gene can create wide adaptation possibilities of diphtheria causative agent.

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