Cells that are in the process division are vulnerable to damage by AC electric fields that have specific frequency and field strength characteristics. The selective destruction of rapidly dividing cells can therefore be accomplished by imposing an AC electric field in a target region for extended periods of time at particular frequencies with particular filed strengths. Some of the cells that divide while the field is applied will be damaged, but the cells that do not divide will not be harmed. This selectively damages rapidly dividing cells like bacteria, but does not harm normal cells that are not dividing. Improved results can be obtained when the field is sequentially imposed in different directions.