Aplastic Anemia Causes

Most articles speculate on benzene and similar toxic chemicals as a possible cause for AA and other immune related disorders, but they also say the disease is "idiopathic" which is a medical term for "we don't know".  

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11882782&dopt=Abstract

A large epidemiologic investigation of a benzene-exposed population has shown that NQO1*2 homozygotes exhibit as much as a 7-fold greater risk of bone marrow toxicity, leading to diseases such as aplastic anemia and leukemia.