What is so unusual about the way retroviruses replicate?

Retroviruses, such as HIV, contain RNA, not DNA. After infecting a host cell, the retrovirus RNA serves as a template for the synthesis of complementary DNA through an enzyme called reverse transcriptase.

The retrovirus usually inserts itself into the host genome, becomes a permanent resident, called a prophage, and is capable of making multiple copies of the viral genome for years. It makes multiple copies for the viral genome for years.

Retroviruses are unusual because they reverse the usual flow of information from DNA to RNA.