Human immunodeficiency virus, or HIV, is a human retrovirus that infects lymphocytes and other cells bearing the CD4 surface marker. The virus is transmitted primarily by sexual and parental routes. There are two ways blood feeding arthropods can spread disease, mechanically, by simple transfer of virus between hosts by contaminated mouth parts, or, biologically, which would require virus replication in arthropod tissues (especially salivary glands). There are some important factors which have proven that AIDS is not transmitted by mosquito bite. These factors are: (1) AIDS virus can not replicate inside the mosquito, bed bug, flea, or other blood sucking insect and the lack of replication of HIV in arthropod cells due to lack of T4 antigen on cell surface, and (2) it is unlikely that HIV is transmitted by insects, given the low infectivity of HIV and the short survival of the virus in the mosquito. HIV appears to be much less easily transmitted probably due to lower titers of virus in body fluids. So, on the basis of experimental evidence and probability estimates, it has been concluded that the likelihood of mechanical or biological transmission of HIV by insects is virtually nonexistent.