Suzanne, Harrison and others in “Unfinished Business”

Without tissue samples, most of this is just educated guessing, but I think I understand how an alien blends with whoever happens to be unfortunate enough to get in its way. It's a combination of osmosis and cell-phase matching. Watch. Through osmosis, alien cells invade the human body. These cells then spread out, seeking human cells that they can bond to. This bonding allows all the genetic information from the individual human cells to communicate to the alien cells.

Killing the human cells in the process?

Yes, but apparently not before it absorbs all of the intelligence of the human victim.

We're talking Night of the Living Dead here, folks. Not only do these guys get to pick our brains; they also get to use our bodies as a perfect disguise.

I don't call open sores and radiation sickness a perfect disguise. I mean, it's not exactly difficult to spot these things in a crowd.

For now, maybe. But it's only a matter of time until they figure out a better way to protect themselves, and conceal themselves.

If any more of them are still around.