Eosinophils are a type of white blood cell, accounting for only 0.5%-5%. Eosinophils in the blood have a distinct diurnal cycle of fluctuations, with a decrease in the early morning and an increase at midnight, with a difference of up to 40% or more. If patients are examined early in the morning, they may have a low percentage of eosinophils. When the concentration of glucocorticoids in the blood is increased, hormonal fluctuations in the body of patients with long-term application of adrenocorticosteroids may cause periodic fluctuations or decreases in eosinophils. In addition, the eosinophil percentage may be low in cases of typhoid fever, paratyphoid fever, major surgery, and burns, mainly due to a decrease in eosinophils in the peripheral blood. Eosinophils have a weak and slow phagocytic capacity and therefore have little to no bactericidal effect and do not play a major role in the antibacterial and anti-infective process. A low eosinophil percentage is not a diagnostic basis for the disease, so patients need to be seen by a physician in a timely manner to confirm the diagnosis and to undergo targeted treatment under medical supervision. If, in addition to eosinophils, the patient also has low neutrophils, lymphocytes, monocytes and other whole blood cells, he or she can take leukocyte-raising drugs as prescribed by the doctor, but the main treatment is to actively cure the primary disease.