Hypersex Symbiosis

It started on a cold, cloudy, winter afternoon. Snow was falling in a thick flurry that day, and the clouds formed a solid dark curtain against the sun. If it hadn't been for those clouds and snow, someone might have noticed a small meteorite streak through the sky and crash into the earth. But it was not noticed. It was small enough that when it had entered the atmosphere it was assumed it would burn up, and was ignored. But it did not burn up, and it was not noticed by governments. Perhaps if it had then things would have been different. Perhaps if it had not been snowing, or the clouds had not been as thick it would have been reported; but it was not.

It was found, not by government agents trained to recover aliens specimens with special equipment, but by someone else, someone who was not as careful and prepared as they should have been. And so the aliens were allowed to find hosts and reproduce, while those would could have stopped them were unaware.