For the past week and a half, the Madison County Health Department has been monitoring an increase in positive tests for Salmonella, according to department spokesperson Christie Green. Six cases have and are all of the same strain, she said. "Epidemiologists believe the cases may be a cluster with a common source of infection," Green said. "However, the source remains unidentified," and health officials continue to investigate. On Thursday, the department instructed local health care providers to be on alert for Salmonella symptoms among their patients and to test those who have symptoms.