More than 50 million Americans suffer from allergies, but the root cause is not always pollen, peanuts, and pet dander. These are some of the most common allergy triggers, but because an allergy is simply an immune response to a harmless substance, a person could conceivably be allergic to anything — even water or bodily fluids. Here's a look at some of the strangest allergies ever documented and how people cope with them.

1 of 10 All Food and Drink jupiterimages. This medical condition is so rare that science has yet to name it, but there are children who are so hypersensitive to food and drink that water is the only thing they can safely consume. One of the most severe and highly publicized cases is that of Kaleb Bussenschutt, a 6-year-old Australian boy who can consume only water, ice and one brand of lemonade. If he eats anything else, he develops ulcers and experiences agonizing stomach pain, so he must receive necessary nutrients through a feeding tube connected directly into his stomach 20 hours a day. Doctors are baffled by Kaleb's case, but they think he suffers from multiple food allergies and severe malabsorption that makes his body unable to cope with eating food. Food allergies affect about one in 20 children, but the severity of Kaleb's allergy is extremely rare.

2 of 10 Cellphones jupiterimages. Allergists and dermatologists are seeing an increasing number of "cellphone rashes," itchy, red bumps or painful blisters along the jaw, cheek and ear. But while it's often referred to as cellphone allergy, it's actually a nickel allergy. Increased use of cellphones has led to prolonged exposure to nickel, a metal that's often used in phone buttons, LCD screen frames and headsets. However, there are also people who may be allergic to the electromagnetic emissions from cellphones. Nickel allergy affects about 17 percent of women and 3 percent of men — women develop cellphone rash more often because they're more likely to have been sensitized to nickel through ear piercing. If you develop a rash from touching money or from skin rubbing against belt buckles or the button of your jeans, you may also have a nickel allergy.

3 of 10 Sex jupiterimages. Believe it or not, there are people who are allergic to sex — kind of. This rare allergy is actually an allergy to a man’s seminal fluid, a condition known as human seminal plasma hypersensitivity. Symptoms include burning sensations, rashes and welts, which is why the allergy can often be misdiagnosed as a sexually transmitted disease. The only real signifier of a semen allergy is that that symptoms appear within minutes of contact. Between 20,000 to 40,000 U.S. women may experience human seminal plasma hypersensitivity, according to Dr. Jonathan Bernstein of the University of Cincinnati, but the cause of it is still being studied so it’s not clear if one person would be allergic to all men. Treatments range from always using condoms to desensitization by exposing a person to small amounts of semen and gradually increasing the amount as the body becomes accustomed to it.

4 of 10 Kissing jupiterimages. While a person can't be allergic to the act of kissing, a passionate liplock can be the kiss of death for someone with severe food, cosmetic or medicinal allergies. Close physical contact and the sharing of saliva can cause chemicals or food particles to pass between parties, which can be deadly if one person is allergic to peanuts and the other just ate a peanut butter sandwich. Such was the case of 17-year-old Jamie Stewart who went into anaphylactic shock and had to be hospitalized after kissing a co-worker who'd just eaten peanuts. About 11 million Americans have food allergies, and as many as 200 of them die as a result of those allergies each year, according to the Food Allergy & Anaphylaxis Network. So what do you do if you love shrimp cocktail but your significant other is allergic to shellfish? Brushing your teeth or chewing gum afterward won't help, and doctors say the only real way to prevent a potential allergic reaction is for both of you to skip the food altogether.

5 of 10 Water Photo: By muratart/Shutterstock Considering the human body is about 60 percent water it seems implausible — if not impossible — to be allergic to water, but about 40 people worldwide have been diagnosed with the condition. Known as aquagenic urticaria, it causes the skin to break out in painful hives and welts when it comes in contact with water, and in severe cases, a person's throat can swell when they drink water. Michaela Dutton (pictured) a British woman who developed a water allergy after the birth of her son, drinks Diet Coke to survive — her body tolerates it even though it contains carbonated water. How do people like Dutton bathe? Quickly and carefully because even brief contact with water can cause a painful itchy rash that lasts for hours. Doctors aren't sure what causes water allergy, but some have hypothesized that elevated histamine levels play a role.

6 of 10 Cold jupiterimages. People with cold urticaria are allergic to cold temperatures, and exposure to chilly air or frigid water can cause their skin to turn red, swell, itch and develop hives. While sucking on a popsicle or strolling through the frozen food section can be uncomfortable for someone with such an allergy, an activity like swimming in cold water can evoke a severe, whole-body reaction — leading to fainting, shock or even death. The cause of cold urticaria isn’t clear, but some people may have overly sensitive skin cells due to genetics or illness.

7 of 10 Heat andy_carter/Flickr. Just as you can be allergic to cold, you can also be allergic to heat. People with heat urticaria develop itchy, red, swollen skin and welts when they’re exposed to temperatures above 109.4 degrees Fahrenheit. These reactions typically occur from an increase in body heat brought on by exercise, hot showers, saunas, blankets or even spicy foods.

8 of 10 Exercise jupiterimages. If someone tells you they’re allergic to exercise, there’s a slim chance they’re telling the truth. About 1,000 people suffer from exercise-induced anaphylaxis, a severe allergic reaction caused by physical exertion that may be related to eating certain foods or taking specific medications before exercise. Fatigue, itchy skin, hives and swelling of the neck, torso and extremities typically appear during or right after exercise, and symptoms can linger for hours. More severe reactions can include choking, vomiting, increased blood pressure and even death, so people with this allergy are advised to carry an epinephrine kit.

9 of 10 Touch Wikimedia Commons. People with dermatographia are sensitive to pressure and touch, and depending on the severity of the case, even a slight touch can trigger an allergic episode that creates a raised, itchy red rash. It is unknown why this allergy occurs, but it's estimated that 2 to 5 percent of the population has it. Because words and designs can easily be scratched on the skin, the condition is often referred to as “skin writing disease.”