Answer:
A retrovirus is any of a family of RNA viruses that have an enzyme (reverse transcriptase) capable of making a complementary DNA copy of the viral RNA, which then is integrated into a host cell’s DNA. The family includes a number of significant pathogens, typically causing tumours or affecting the function of the immune system, e.g. HIV.
One of these retroviruses is Human Immunodeficiency Virus, or HIV. This is the virus that causes Acquired Immunodeficiency Syndrome, also known as AIDS.