Understood, thank you. That makes sense.

It was a great question, and a great reason to make an HN account too!

I guess when the aliens finally arrive, the important part will be to avoid hugging them.

Probably safer to hug them than another human. Most diseases don't even cross species boundaries, seems unlikely they'd do well trying to infect a completely alien being.

The reason that diseases from the old world were so deadly, when they crossed the pond, is that they'd evolved to spread quite well even in humans with an evolved or acquired resistance and were suddenly in a completely unprepared population.

The danger would be organisms that simply eat our biochemistry but are entirely immune to any of our defenses. Think flesh eating bacteria from space, only it also basically eats our entire biosphere.

Aliens from different biospheres will never be able to have physical contact.

If they are close in biochemistry the dangers are extreme to catastrophic. If we actually found a crashed UFO with bodies we should drop a thermonuclear bomb on it immediately. Just ask the Native Americans.

If they are not biochemically close chances are we and our environment would be horribly toxic, freezing, or boiling hot to them and vice versa.

People in the new world and old world were both human, so diseases were able to spread.

Aliens probably won’t be human, so their diseases infecting us is unlikely at first

Username checks out ;)