Final answer:
The +ssRNA virus genome can directly act as mRNA because of its positive polarity, which makes it immediately ready for translation by the host ribosomes into viral proteins.
Step-by-step explanation:
The genome of the +ssRNA (positive single-strand RNA) virus can act directly as mRNA and be translated into viral proteins by host ribosomes without the need for transcription or reverse transcription. This is due to the positive polarity of the +ssRNA, meaning it is already in the correct orientation to be read by the ribosomes, akin to cellular mRNA. The RNA of retroviruses, ssDNA viruses, and dsRNA viruses needs to undergo different processes before serving as mRNA for protein synthesis.