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There are two retroviruses in the oncoretroviridae group which cause human disease; these are type I and type II human T-cell lymphotropic virus (HTLV).
HTLV was the first human retrovirus to be isolated. It is spread by sexual contact, from mother to child and through contaminated blood products.
HTLV-I is endemic in southern Japan, equatorial Africa and South America.
HTLV is able to avoid clearance by the host's immune system by remaining in the latent state, as a provirus within the host's genome.