Ever felt an unaccountable sense of fear and futility? Turns out you should have been even more terrified than you were. A "sense of impending doom" is an actual medical symptom for some very serious conditions.
The Sweats and Impending Doom
Europe had quite a few problems during the late Middle Ages. There were wars, independent brigands, religious revolutions, and, of course, many different kinds of plague. One disease, known as the Sweating Sickness, would take people from perfectly healthy to dead over the course of one day. It struck Anne Boleyn during the height of her courtship with Henry VIII, and if she hadn't survived it we would have had an entirely new history, and an entirely different sexy historical monarchy to make salacious tv series about. Its fatality alone would have made "the sweat" terrifying, but it was made almost supernatural by an eerie quirk. The first symptom anyone had was a strong sense of impending doom.
The sense of impending doom did not disappear into history the way the Sweating Sickness did. People who suffer from depression or anxiety often have it, of course, but it's also associated with ailments entirely separate from emotion. A sense of impending doom is a symptom of anything from cardiac arrest to a jellyfish sting. It's not conclusive in and of itself, but it is listed as one of the identifying features of multiple medical problems.
Modern Medicine and Impending Doom
It's not surprising that a person having a heart attack would have a sense of impending doom. Pain and pressure on a the chest, shortness of breath, and heart palpitations, even when they're subtle, are enough to depress someone's spirits. Less explainable is the sense of impending doom that pops up after being stung by a jellyfish. The irukandji jellyfish is tiny, but powerful. Generally, people who get stung don't die, provided they get out of the water before the pain kicks in and immobilizes them, and provided they get basic medical treatment. Patients undergoing treatment of irukandji stings are understandably distressed - the stings are maddeningly painful - but they're also certain they're going to die. Sometimes they're so certain, and so oppressed by the feeling, that they ask their doctors to kill them to end their suffering, as they "know" that they're never going to get better.
The strangest cause of a sense of impending doom is in your spice cabinet. Nutmeg, by the tablespoonful, is sometimes used by people to get high. Too much nutmeg can provoke hallucinations, heart palpitations, and yes, a sense of impending doom. People have been convinced they're going to die via pie seasoning.
All of these ailments seem to have certain things in common. People experiencing a heart attack have an irregular heartbeat and have difficulty breathing. Nutmeg is a anticholinergic - a substance that blocks the neurotransmitter in charge of the sympathetic nervous system. This system regulates the unconscious processes in the body, like digestion, breathing, and heart rate. Anticholinergics aren't a popular class of drugs, as they mostly make people feel sleepy, but too much of them will also mess with heart rate and breathing. The irukandji jellyfish venom, though imperfectly understood, contains catecholamine, a substance that also affects the sympathetic nervous system.
Although we don't consciously control our need to breathe or our heart beat, it seems we can feel it, on a certain level, when they are in trouble. A sense of impending doom may be a system we generally take for granted screaming at us to do something. So if you feel a sense of doom, especially if you've recently been swimming near Australia, or eaten to many pieces of spice cake, consider consulting a doctor.