When Europeans arrived in the Americas and Australasia they unintentionally introduced a heap of diseases that the locals had no immunity to. 90% casualty rates and societal collapse followed.
During Operation Revival you get two isolated populations meeting each other.
The difference here is that Clan populations have only been away from Spheroid populations for a couple hundred years, while the populations of Europe, the Americas, and Australia diverged tens of thousands of years ago.
Anything is possible. But it seems unlikely that the microbes carried by Clanners to the Pentagon Worlds and Kerensky Cluster would have evolved into something highly contagious and deadly to Spheroids after so short a time.
(That said, it would be a nifty explanation for why Wolf's Dragoons was so successful. It wasn't the Clan training, the old SLDF designs, or Jaime's tactical genius. It was the Pentagon Pox!)
Given the nature of the Home Worlds
In-universe, this seems more likely. There's some Kerensky Cluster Fever native to those worlds that all Clanners got exposed to early in the settlement of those worlds but devastates Spheroid populations as they recontact and trade microbes.
Out-of-universe, the likelihood that extraterrestrial microbes would be compatible with Earth life probably approaches nil (unless both biologies come from the same panspermia). As Carl Sagan used to say, you will be more closely related to the tree in your front yard than any extraterrestrial lifeform.
the Clans technological advantages, and the Star League having already provided fixes for most IS diseases I think we have to say that the Clans have the advantage here.
The Society could certainly field some impressive viral genetic engineering...