Pain, fever, diarrhea, coughing, vomiting--these are all conditions most of us wish did not exist. We go to the doctor to get relief. So why does evolution keep them around? These miseries may actually help us survive by protecting our bodies from the damage of infection, injury, and toxins.

No one wants to feel pain, yet pain helps keep us alive. Individuals with a rare condition called congenital insensitivity to pain often injure themselves unintentionally, sometimes with devastating consequences, such as bone infections or destruction of tissues and joints.

Fever is similar to pain in that it makes us feel terrible, but can be beneficial. It provides us with a defense against infection by boosting the immune system and fighting off heat-sensitive pathogens. Given all the good that seems to come from fever, Dr. Matthew Kluger of the College of Health and Human Services at George Mason University suggests fever is most likely an adaptive response.

Kluger's studies show that animals that experience lower or no fever with infection fare worse than those whose temperatures shoot up. When he infected lizards with heavy doses of live bacteria, all those that experienced fever survived, while those that couldn't raise their body temperature died. The results of other studies, compiled by Dr. Sally Eyers and colleagues at the Medical Research Institute of New Zealand, found that the risk of death was higher in animals given fever lowering medication.

While we have a fever, our bodies strategically employ a lot of other tools too to fight infection and get us healthy. "An infected animal loses food appetite, does not want to interact with anyone else, increases his body temperature, and fights infection," notes Kluger. "Then when infection is fought off, you see a change in behavior." In his fever studies, "before even looking at the temperature recorder," he explains, "we could see when fever broke."

Even the least glamorous symptoms can have a silver lining. Studies have shown, for instance, that individuals infected with bacteria that cause diarrhea actually stay sick longer when they take anti-diarrhea medications than when they let nature take its course without meds. The same can be true for coughing: In one study, elderly patients with a less-sensitive cough reflex were more likely to get pneumonia than their coughing cohorts.

The argument extends to vomiting, too, particularly during pregnancy. Some researchers argue that morning sickness is an evolutionarily acquired defense to protect a pregnant woman and her fetus from dangerous food-borne toxins. Across the world, nearly 70% of women experience nausea and vomiting during pregnancy. Many foods, especially meats, may contain viruses, bacteria, or fungi that could be dangerous to humans in general, but some are more vulnerable than others. Dr. Paul Sherman of Cornell University argues that the developing embryo and carrying mother are especially susceptible to the negative effects of these pathogens because of their weakened immune systems.

A pregnant woman's immune system is suppressed during pregnancy to prevent the body from rejecting the fetus. The fetus is especially vulnerable during the early stages of pregnancy because that is when it is growing and developing most rapidly. If a woman became ill from food borne toxins, especially in her first trimester, it could result in birth defects or miscarriage. Compiling nine different studies, Sherman found that women who experienced nausea and vomiting during pregnancy were less likely to miscarry compared to those without those symptoms. Though much more research needs to be done, it seems that morning sickness may be a defense evolved to protect the pregnant mother and her growing fetus.

Does this mean that we should all rid our medicine cabinets of anti-nausea pills, painkillers, fever lowing medications, and cough suppressors? Dr. Randolph Nesse of the University of Michigan, one of the founders of the field of Evolutionary Medicine, suggests that in many cases it could still be safe to turn off (or tone down) the body's more disagreeble defenses. What we have to do is better understand the system so that we know when it is not safe to do so.

For Nesse, the body's defenses are akin to a smoke detector. "The system is set to go off like a smoke detector very often when there's no fire," Nesse explains. A smoke detector will alarm when it senses fire or smoke just as the body's defenses kick in when they sense a danger to the body. Sometimes the detector will get it right, but often times it will go off when there's no real threat. As the saying goes, nothing in life comes free. There is a cost to the defenses your body elicits against a sensed threat. This cost is relatively small compared to the cost of not defending the body if something actually is wrong. Pain is uncomfortable and costs energy, but if you did not feel pain when you broke your leg, you would be in even bigger trouble.

Nevertheless, most of the time, defenses kick in when they are not needed. "Many communities prohibit parking adjacent to a fire hydrants," Nesse points out, "although the chance that a fire truck will use that hydrant on a given day is less than one in 100,000." Similarly, "Birds flee from backyard feeders when any shadow passes overhead," even though most of these shadows do not present a real threat to the bird. The frequency of false alarms is why you can block pain or fever most of the time and not see really bad things happen. "If [medical professionals] have an understanding of the smoke detector principle," Nesse explains, "they can begin to decide when it's safe to block defenses and when it's not."