If you find yourself in a cold sweat just thinking about your annual flu shot, you can blame evolution, Cynthia Murray writes in The Conversation.

While most people dislike getting pricked by a needle, some experience far more serious emotions. About 3 percent of the population has a phobia about the sight of blood, sustaining an injury, receiving an injection, or undergoing other medical procedures.

The fear can be hereditary, says Murray, a clinical neuropsychology registrar and research assistant at Neuroscience Research Australia. A 1998 study found a high concurrence of blood-injection-injury phobias among twins and other immediate family members. These phobias produce far more than mere discomfort: The patient's heart rate spikes and then drops rapidly, which can cause nausea, sweating, and even fainting.

Needle free vaccines? 'Nanopatch' discovery may bring us closer

Curiously, though, those symptoms are different than those produced by other phobias. For instance, when people who have phobias of heights or spiders confront their fears, their blood pressure and heart rate rise and stay elevated—priming a person to attack or make their escape, rather than to pass out.

So why are you more likely to faint when faced with a needle than with a 20-foot-drop?

Inconclusive research

According to Murray, one hypothesis suggests that fainting increases a person's odds of survival. After a sharp-object injury, lower blood pressure means less blood loss.  

Another theory is that blood-injection-injury phobias are actually an evolved version of disgust, which is associated with vomiting and nausea. But that idea doesn't track with our understanding of the purpose of disgust, Murray writes. Disgust is thought to protect mammals from diseased foods: It's the reason you don't want to drink spoiled (and smelly) milk.

Another theory suggests that fainting could have been an asset to our early ancestors when faced with inescapable threats. In that sense, fainting when seeing a needle—which our brains perceive as a life-threatening danger—is equivalent to "playing dead."

While experts normally recommend you relax when facing a fear, a blood-injection-injury phobia may call for a different approach. Relaxation techniques, such as deep breathing exercises, could cause your heart rate to drop even further, making a fainting episode more likely.

Instead, if you find yourself woozy over your inoculations, engage in conversation, clench your fists, and maintain as much tension in your body as possible, Murray recommends. Keeping your heart rate up may keep you upright as well (Murray, The Conversation, 4/27).