Control of infection classically involves hand and healthcare hygiene, reduction of selective and ineffective chemotherapy, reduction of invasive procedures and achlorhydria and adequate staffing, along with appropriate containment and concentration of patients. Investigation and control of any continuing sources of infection in food and water supplies is important also, as is recognition of individuals carrying high-risk strains and species. The onset of infection may be distant from the time of acquisition and may critically affect epidemiological assessment of control points. Carriage may be prolonged, increasing the likelihood of recurrent infection and exacerbating the difficulty of control. Mortality associated with resistance is difficult to assess retrospectively and may not be high, complicating analysis of the success or failure of control measures.