I agree on the scarcity of ET civilizations, in both space and time. My guess is about 1 intelligent society per billion stars at any one time. This would put the nearest ET to us several thousand light years away, on the average.

I think the possibility of Terran and alien organisms being able to infect each other is highly unlikely. Even on Earth, where all organisms are chemically closely related, only a very few microbes can actually infect us. On another world, the life would have evolved so differently that even if it was very similar in chemistry to us it would have little in common with us in detail. I.e., our bugs wouldn't know how to attack them, and vice-versa. Likewise, I doubt alien organisms would be useful as food, without extensive processing. they might even be poisonous, even if the basic constituents were identical to ours.

OTOH, I do believe alien proteins would be similar enough to us that they would be recognized by our bodies as a potential hazard, and there would probably be powerful allergic reactions in each towards the other. Earth crops and bacteria woul have very little chance of growing there, but if any could gain a foothold, they would quickly displace the local biota. Unfortunately, I believe the only way we could transform another world into an earth-like paradise would be to overwhelm it with genetically engineered organisms and sterilize the local wildlife.

But there's no need for that. Any technology capable of interstellar travel wouldn't need to start farming when it got to its destination. It would be able to manufacture food in chemical factories. We don't need earthlike worlds, all we need is matter and energy, and there's plenty of that in our own system.