Ragged Wings

I was looking at some close up pictures I had taken of insects, and noticed that the wings, while they always had a strong, smooth line on their front edge, they (almost) always had uneven or outright jagged back edges.
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Often they are damaged, but even when they are not the back edge of an insect wing is almost never a single smooth line.

The same with bird wings; the fan of feathers at the back of the wing contrasts with the smooth curve of the front edge.

I don’t know aerodynamics at all.  But I’ve seen this so often that I wonder if there is some advantage to having a ragged edge on the trailing side of a wing.

Does a serrated rear edge reduce drag, or does it provide stability?  Would airplanes benefit from having a serrated rear edge instead of a straight edge?  Or is this related to the motions that insect wings go through?  Is the ragged edge just an artifact of how wings grow?  Or is this just confirmation bias on my part?  Or has natural selection discovered a consistent advantage?

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Alien Virus

First question!

One of the things authorities have said about the meteor that exploded over Russia this morning is that “…Neither is there a real risk of alien death viruses.”

I’ve always wondered, when the concept of a completely alien virus (or microbe) comes up in fiction or in science, would we have no defenses against such a virus, or would the virus have no way to infect our biology?