1. Dying some horribly painful, prolonged, and horrific death. Like... in the middle of an ocean, and being eaten by some long-extinct gigantic marine reptile. (Very realistic this one, but you did ask. )
More immediate and likely fears:
2. Vomiting (especially at night, when the dark makes me feel even more trapped)
3. Blood draws
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"There are surely other worlds than this -- other thoughts than the thoughts of the multitude -- other speculations than the speculations of the sophist. Who then shall call thy conduct into question? who blame thee for thy visionary hours, or denounce those occupations as the wasting away of life, which were but the overflowings of thine everlasting energies?" ~The Assignation, Edgar Allan Poe.