We don't really understand viruses that well. Viruses appear to have many tricks up their sleeves, including air transmission. Earth is full of viruses (thousands upon thousands), many of which we do not yet understand or even know exist. We could certainly develop (consciously or accidently) a bio-weapon that changes how they behave too. If we ever meet a civilization, we're not just going to say "hello" and then go our separate ways. We're going to come in close contact with them, their animals, and technology. This close contact increases the chance of cross-contamination. The chances are very high that they could introduce a virus into our ecosystem or we could introduce a virus into theirs. An alien virus is perhaps the biggest threat we face from a non-hostile alien civilization.

You bring a good point. If we stay apart, then the chances are small because of the distances involved. But let's consider the following scenario. An alien civilization uses FRBs to communicate with us across a vast distance. They teach us how to create an "FRB 3D printer," so we can share information. A malicious civilization could use this FRB 3D printer to send a virus across vast distances without having to set foot here. This would be a good way to transform our planet and invade it at a later date.