Let me post an excerpt from Battlefleet Gothic for your edification.
“A fatal error, ‘twas, listening to that damned old fool. We were carrying virus bombs for the planet below, but Hergol told us since we were in a jam, we might launch a few at the things in space, only it didn’t do a thing to ‘em.
Well, so we thought until the damned things rammed us a week later and got hold of the ship. They spat this acid, this burning spittle everywhere, and within and hour, those that didn’t die from the burns were sick as hell with the same virus we’d hurled at the beast to begin with.”
TL;DR: Nids will just adapt their shit and then use the virus tech against you. And the hive mind has a nasty sense of humor.