Human T-Lymphotropic Virus (HTLV) is a retrovirus associated with depression of the immune system. Several different retroviruses are classified in the HTLV family, including HTLV-I and HTLV-II. The differences between the various HTLVs are primarily related to geographic distribution and the precise effects of the virus on the body. HTLV is more common in the developing world, with several different blood tests available to detect the presence of HTLV antibodies in the blood.

This retrovirus was first discovered in Japan in 1977, and isolated at the National Cancer Institute in the United States. Initially, early work on HIV, the virus which causes AIDS, classified HIV as a member of the HTLV family, and the virus was briefly known as HTLV-III. However, additional research showed that HIV acted very differently from HTLV, and that it belonged in an entirely separate classification. You may still see HIV described as HTLV-III or LAV, a French term for the virus, in old texts.

HTLV is linked with a number of health problems, including demyelinating diseases which affect the central nervous system, such as tropical spastic paraparesis, along with adult T-cell leukemia and lymphoma. Viruses in this group work by initially stimulating the immune system, which ultimately leads the immune system to go into overdrive, at which point it starts attacking itself. As the immune system becomes weaker, the patient is at risk of opportunistic infections which may take advantage of the vulnerable body.

Once infected with HTLV, someone carries the infection for life. Treatment is primarily focused on managing the conditions associated with HTLV, since no cure has been developed. In some cases, people may carry the retrovirus without developing any symptoms, often passing it on to others because they are unaware. Other people develop cancers and other diseases, depending on the form of HTLV with which they are infected.

This retrovirus can be transmitted in a number of ways. It is carried in blood and some bodily fluids, so instances of blood to blood contact, sexual contact, and shared needle use can result in the transmission of HTLV. HTLV is also carried in the breastmilk. Widespread HIV/AIDS education often helps to reduce HTLV infection rates, as the same techniques used to avoid AIDS can also be used to reduce the risk of HTLV infection. However, in regions without adequate education or funds to provide preventative measures like needle exchanges and condoms, HTLV infection rates tend to remain consistent, or to increase.