Sure, I get you. There are all kinds of possibilities in a vast universe. If, for the sake of discussion, we can assume that alien bacteria and viruses have roughly the same physical constraints — it's pretty unlikely you see that kind of adaptive behavior toward multicellular organisms. There is a physical limit on the size of bacterial genomes and viral RNA is a single set of instructions that get inserted into the host's own genome, so they literally can't be that complex. Every bacteria and virus on Earth targets a specific host, environment and/or life cycle - they have to specialize and adapt to survive.

That's not to say we couldn't inadvertently pick up a bioengineered complex organism from an advanced alien species - that would be a completely different story.