The Mississippi Department of Health said in a news release that the new case occurred in an adult in Calhoun County.
Earlier, two positive cases were reported in Leflore County with one each in Coahoma and Scott counties.
Health officials say there have been two positive WNV mosquito samples reported in Madison and Hancock counties.
Symptoms of West Nile virus may include fever, headache, nausea, vomiting, a rash, muscle weakness or swollen lymph nodes. In a small number of cases, infection can result in encephalitis or meningitis, which can lead to paralysis, coma and possibly death.