I think you are being a bit too optimistic when it comes to close contact with aliens. There would be probably years worth of research on whether such contact would be safe done from both sides as any space-faring civilization would very well know of the risks of viruses, bacteria, and similar things.
Also, a 3D printer capable of printing viruses would have to be almost impossibly intricate as it would have to be capable of literally printing DNA. And their attempt could easily fail because they would have no prior knowledge of our immune systems so it could easily just defeat it. It would be a fifty/fifty shot at either it does nothing or it wipes us out and they would have no knowledge on whether it worked because of the time delay.
There is no guarantee that alien intelligence is similar to ours, so there's no telling what they would do. But let's use human intelligence as an example. Humans are very adventurous and wreckless. For example, we know that there could be dangerous viruses on Mars (if there's life), and yet we're only a few years away from going there in spite of this danger. We'll be digging around in no time. Closer to home, we know that bats and other creatures carry viruses, and we're still poking around in caves. We also know that there could be viruses in frozen lakes in the arctic, and we continue to dig up and bring up life from its icy depths. So, in spite of the danger, we continue exploring and touching other life-forms. A sufficiently advanced civilization may also decide to come poke around on Earth for research purposes.
We are already capable of creating organisms using synthetic DNA. In a few decades, this will be routine. So, a sufficiently advanced civilization (several hundred, thousands, or millions of years ahead of us) could do it fairly easily. We now know that FRBs are capable of carrying more complex information, so if we use them to communicate, we could send complex instructions to each other. You're right that they might not understand our nervous system, but if we share that information then it will be easy. Because of our social nature, humans like to share too much information. Here's exhibit A:
We even put our home address!