Retroviridae
The problem(s) associated with development of a vaccine against HIV is/are

all of these
antibody alone may be insufficient
virus can spread from cell to cell by fusion to produce syncytia
rapid mutation of HIV

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Retroviridae
HIV or Human Immunodeficiency Virus is considered unique because each virus contains:

a genome made up of both DNA and RNA components
more than 100 copies of the viral genome
sticky proteins that allow it to bind tightly to host cells
a viral enzyme known as reverse transcriptase

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