Case fatality rate represents the killing power of a disease.  It is  simply the  ratio of deaths to cases. The time interval is not specified.  Case  fatality  rate  is  typically  used  in  acute infectious diseases (e.g. , food  poisoning, cholera, measles). Its  usefulness  for  chronic  diseases  is  limited,  because  the period  from  onset to  death is  long and variable.  The  case fatality  rate  for  the  same  disease  may  vary  in  different epidemics  because  of  changes  in  the  agent,  host  and environmental  factors.  Case  fatality  is  closely  related  to virulence.