Aplastic anemia (referred to as reblastosis) is a pluripotent stem cell disorder characterized by a lack of hematopoietic cells, replacement of bone marrow hematopoietic tissue by adipose tissue, and a decrease in whole blood cells in peripheral blood, which is often associated with severe clinical anemia, infection and bleeding. Although there is a lack of comprehensive survey in China, according to the results of recent censuses in Pingdingshan City, Henan Province and Mudanjiang, Heilongjiang Province, the incidence rate is 1.87/100,000 and 2.1/100,000 respectively. The majority of patients are young adults, with more men than women. The causes of secondary reoccurrence are as follows: a. Drugs and chemical substances The substances can be divided into two categories according to their effects: one category is related to the dose of toxic substances, as long as the dose of exposure is large, anyone can occur bone marrow regeneration disorders, such as nitrogen mustard, cyclophosphamide, 6-mercaptopurine, Maryland and other anti-tumor drugs; the other category is in the exposure to therapeutic doses (drugs) or general doses can occasionally in some people The other group is the drugs that can occasionally cause reoccurrence in some people when exposed to therapeutic doses (drugs) or general doses, such as chloro(co)-mycin, botulinum, sulfonamides, etc. Of these, chloramphenicol is particularly worth mentioning as the most common cause of drug-induced reoccurrence, which is associated with the abuse of this drug. This drug is an azobenzene derivative, which in most cases causes short-term reversible myelosuppression in relation to the drug dose. The effect is due to the inhibition of protein synthesis in mitochondria in bone marrow cells, and the myelosuppression is often restored after discontinuation of the drug, and in very rare cases (incidence about 1/2 to 40,000 → 1/200,000), the myelosuppression is independent of the drug dose and appears to be a specific reaction to the drug (there may be an inborn enzyme defect leading to a defect in nucleic acid synthesis, or it may be an allergic reaction), even when the contact with Severe myelosuppression persists even after exposure to the drug has been discontinued. POTAXONE can also cause reoccurrence, independent of the drug dose, and is second only to chloramphenicol in its occurrence. Benzene is the most widely used industrial chemical and the most important myelosuppressive toxicant. It appears to cause reoccurrence independent of dose and may be an idiosyncratic reaction, although long-term daily exposure to benzene is a greater risk than a single large dose. Occasionally, myelosuppression can occur months or even years after exposure to benzene has ceased. Second, ionizing radiation Various kinds of ionizing radiation such as X-rays, radioisotopes, etc. can damage the hematopoietic stem cells in addition to the hematopoietic microenvironment and affect the proliferation and differentiation of stem cells. The degree of damage is related to the dose of exposure to nuclear radiation. Infection Recurrent disease can occur after viral hepatitis, and both hepatitis A and B can cause the disease. About 10-20% of young patients have a history of hepatitis before 6 weeks of allogeneic cytopenia. Such cases tend to be more severe. It is usually believed that the virus in question can affect both the liver and the bone marrow, and many of the so-called primary remittent cases have had viral respiratory infections before the onset of the disease. Other cases of reoccurrence may occur during pregnancy and the anemia may be reduced or relieved after delivery. In recent years, a few patients with reoccurrence have been found to have red blood cells sensitive to complement, and a few cases of paroxysmal sleep hemoglobinuria have gradually evolved into typical reoccurrence.