The possible causes of high fever without high white blood cells in routine blood tests are viral infections, mycoplasma infections, elderly patients, severe infections or bone marrow suppression. 1. Viral infections and mycoplasma infections without bacterial infections usually do not lead to elevated white blood cells, but due to viruses or mycoplasma and their toxins into the blood, which affects the thermoregulatory center, leading to an increase in body temperature. 2. Elderly patients or seriously ill patients may not have high white blood cells during bacterial infection due to poor body response, but the thermoregulatory center is also affected and the body temperature is high. When the blood image is rechecked after a period of time, the white blood cells will rise gradually. 3. Some patients have a history of tumor radiotherapy, which is immunosuppressive to the bone marrow, thus leading to a decrease in the body’s ability to resist infection, and the blood picture does not rise with it, or may even fall. If the above situation occurs, it is recommended to consult the hospital in time and be treated under the guidance of the doctor.