A retrovirus is a type of virus that uses RNA as its genetic material. When it infects a host cell, it converts its RNA into DNA through an enzyme called reverse transcriptase. This newly formed DNA can then integrate into the host's genome, allowing the virus to replicate and spread. Retroviruses are known for causing various diseases, including some types of cancer and HIV, which leads to AIDS. They can be challenging to treat because they become part of the host's DNA, making it difficult for the immune system to detect and eliminate them.