Low white blood cells after fever is mostly suggestive of viral infections, the natural course of viral infections is usually 7 to 10 days; the recovery time of white blood cells varies greatly among individuals, but most of them take about two weeks to a month to return to normal. Viral infections can affect the body’s immune system, routine blood tests can appear low white blood cells, neutrophils decreased, while the lymphocyte ratio increased phenomenon, such as influenza virus infection. Viral infections are self-limiting, and the average person can recover in about 7 to 10 days, and the white blood cell count usually returns to normal in 14 to 30 days. However, in the case of elderly and frail people, or people with immunodeficiency diseases, who are prone to complications such as pneumonia and pleurisy, the course of the disease may be prolonged for 1 to 2 months before recovery, and the time for white blood cells to return to normal will also be prolonged. Low white blood cells should be treated under the guidance of a physician.