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 . since the vulnerability of the dividing cells is strongly related to the alignment between the long axis of the dividing cells and the lines of force of the electric field , improved results can be obtained when the field is sequentially imposed in different directions .