Retroviruses are a type of virus that have RNA as their genetic material. They are unique because they can convert their RNA into DNA once they infect a host cell. This process is facilitated by an enzyme called reverse transcriptase, which allows the viral DNA to integrate into the host's genome, potentially leading to long-term infection. One well-known example of a retrovirus is the Human Immunodeficiency Virus (HIV), which can cause AIDS. Retroviruses can be challenging to treat because they can hide within the host's DNA, making it difficult for the immune system to detect and eliminate them.