Many thanks for the compliment. :))

If I'm not wrong, parasitic nucleic acids refer to nucleic acids (DNA or RNA) that are infectious.

We call viruses as obligate parasites because viruses must rely on the host cell's machinery to reproduce more virions. So parasitic nucleic acids would mean DNA/RNA that hijacks a cell's replication machinery. And only viruses do that, as far as I know. (I initially thought prions could do that as well, but apparently prions are infectious proteins. Prions don't have nucleic acids.)

So, in essence, parasitic nucleic acids simply mean viruses.

As for the pleomorphism, I think it means influenza proteins' structures are highly variable. This means that influenza viral proteins may be harder to be detected by the host's immune cells. Perhaps this also explains why flu vaccines are difficult to make as scientists have to find an influenza viral protein that has a stable structure that the immune system can detect and attack.

And based on the paper you referred to, I'm not sure if the influenza virus pleomorphism is related to host variations. Since the influenza virus assembles its virions from a combination of its own and the host's proteins, I think host variations may contribute to some extent.