Final answer:
CAE and OPP are caused by retroviruses.
Step-by-step explanation:
CAE stands for Caprine arthritis-encephalitis and OPP stands for Ovine progressive pneumonia. Both diseases are caused by retroviruses.
Retroviruses are a family of RNA viruses that have the unique ability to reverse transcribe their RNA genome into DNA. This reverse transcription step is catalyzed by an enzyme called reverse transcriptase, which converts the viral RNA into a DNA copy that can be integrated into the host cell's genome. Retroviruses are known for their ability to establish a long-term, persistent infection in the host.