Regression fever is an acute infectious disease caused by insect-borne spirochetes, characterized by periodic high fever with generalized pain, hepatosplenomegaly and bleeding tendency, and jaundice in severe cases. Depending on the vector, there are two types of tick-borne regressive fever (epidemic regressive fever) and tick-borne regressive fever (endemic regressive fever). The following is an introduction to what can cause the disease. 1, the source of infection: the only source of infection of louse-borne regression fever is the patient; tick-borne regression fever is the main source of infection of rodents, patients can also be the source of infection. 2, the transmission route: the transmission of tick-borne regression fever to body lice and head lice as the transmission vector. The vector of tick-borne regression fever is different kinds of soft ticks. Ticks can carry spirochetes for life. They can also be transmitted via eggs. Therefore, ticks are not only vectors, but also storage hosts for pathogens. 3, susceptible people: men, women and children are susceptible. Immunity does not persist after the disease. There is no cross-immunity between the two types of regression fever. Most patients have increased total leukocyte count, up to 1.5-2×1010/L, and increased neutrophils. Platelets and clotting time are mostly normal, but may be abnormal in severe cases. Serum alanine aminotransferase is often elevated, and serum bilirubin may be increased. There is a small amount of protein, tubular type and red and white blood cells in the urine. Cerebrospinal fluid pressure is slightly increased, with a mild increase in protein and lymphocytes. Spirochetes may be found in blood or bone marrow smear staining microscopy or dark field examination during the febrile period.