The ship's crew died from exposure to our bacteria. They had no defense against it. The siren was an automated sickbay, capable of changing its form according to who needed it. Since the crew died, it instead started taking care of humanity, and thus took the form of a human. The singing was an anesthetic, the mark was a tissue sample, but since she didn't know how to heal a human she instead plugged everyone up to life support.
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