It starts innocently enough. Maybe you’re stretched out on a beach towel, cheering at a baseball game, or spending a little too long pulling weeds in the garden. Hours later, you catch your reflection in a mirror and realize your skin looks different. It might have turned toasty brown. It might be bright red. Sometimes, it’s a little of both depending where you look.

These summertime skin souvenirs are so common that most of us hardly think twice about them. A tan is often treated as a sign of time well spent, while a sunburn is a painful mistake to be avoided. But what exactly separates the two? Why does an afternoon in the sun sometimes leave us bronzed, but other times burned?

Turning Up the Tint

At first glance, a tan and a sunburn may seem like very different outcomes. But they’re actually closely related responses to the same thing: ultraviolet (UV) radiation from the sun.

When UV rays strike your skin, they can do serious damage to your DNA and other important cells and tissues in the body. To help protect you, your body ramps up production of a pigment called melanin, which gives your skin its color. Melanin acts like a natural shield, absorbing and dispersing some of the incoming UV radiation before it can wreak more havoc.

As melanin first builds up during your gardening session or at the ballgame, your skin darkens. This is what gives you a tan. But melanin can only do so much. If the amount of incoming UV radiation exceeds your skin’s ability to protect itself, damage begins to accumulate. That’s when a sunburn occurs.

And sunburns do more than just ruin your vacation photos. They’re actually an inflammatory response — your damaged skin cells are releasing distress signals that trigger increased blood flow to the now lobster-like area. This defense mechanism causes the redness, warmth, and pain that may have you reaching for the aloe vera. A severe sunburn is also colloquially referred to as sun poisoning. You’re not being poisoned, but your body’s response is strong enough to cause nausea, chills, fever, and other intense symptoms.

The time it takes to tan versus burn depends on a variety of factors, including how long you’ve been basking in the sun, how intense the UV is that day, and your genetics (pale skin will burn much faster than darker skin). Because UV light, which isn’t visible, passes through clouds, you can even tan or burn on an overcast day.

It’s worth noting that a tan doesn’t mean you’ve escaped any sun-related harm. Dermatologists consider a tan to be evidence that UV exposure has already caused skin damage and raises your risk of skin cancer (sorry). That means, unfortunately, that there’s really no such thing as a “healthy” tan.

Sunscreen can help block some UV particles and reduce skin damage, but at the end of the day (when the sun has set), it’s your skin that has been hard at work keeping you healthy. (Covering up with a hat and long sleeves while outdoors, of course, makes the job a lot easier.) But whether you leave the sun with a golden glow or a painful burn, both are reminders that you can have too much of a good thing — especially when it comes to sunlight.

Anyone who’s watched a TV medical drama can tell you that blood type matters when it comes to getting a blood transfusion. And yet more Americans know their zodiac sign or credit score than their own blood type, despite the latter having a consequential impact on your health — one that goes far beyond your veins. 

Scientists have been studying the connection between blood type and disease for more than a century, and they’ve discovered fascinating associations with heart disease, certain cancers, cognitive issues, kidney stones, infectious diseases, and much more. It’s remarkable to think that the letters on your blood donor card could be quietly influencing your body in ways you never expected.

Blood Types, Explained

Human blood is categorized into four main groups: A, B, AB, and O. These groups are based on whether certain molecules called antigens are present on the surface of red blood cells. People with type A blood have A antigens, people with type B have B antigens, people with AB have both, and people with O have neither.

Blood is also labeled positive or negative depending on whether it contains a protein called the Rh factor. That’s why there are a total of eight common blood types, including A+, A-, B+, B-, and so on. (There are also other, rarer types — only about 50 known people in the world have the Rh null blood type, for example.)

These antigens are important to know because our immune systems are sensitive to them. If someone receives incompatible blood in a transfusion, their immune system may identify those antigens as foreign and attack the donated cells. This can result in fever, low blood pressure, kidney failure, and even death.

These blood-group molecules don’t just exist in blood. Researchers have found them in tissues throughout the body, including the digestive tract. Their widespread presence may help explain why blood type appears connected to certain diseases and other health issues.

Why Some Blood Types Clot More Easily

One of the most consistent findings on the link between blood types and health involves clotting. Research has repeatedly shown that people with blood types A, B, and AB have a higher risk of developing dangerous blood clots than people with type O blood. One large study published in 2020 found that people with type A or B blood were significantly more likely to develop deep vein thrombosis — a clot usually found in the legs — as well as pulmonary embolism, which occurs when a clot travels to the lungs.

Scientists think a clot-promoting protein called von Willebrand factor may help explain the connection. Higher levels of the protein may make clotting more likely, and folks with type O blood typically have less of it.

These same clotting mechanisms may also help explain why non-O blood types have been associated with higher risks of heart attack, coronary artery disease, and stroke. Some studies suggest that people with AB blood — those with both primary blood antigens — may face the highest cardiovascular risks of all the blood groups, though researchers are still investigating why.

That’s not to say having an O blood type is always preferable, though. One large Swedish study found that people with O blood types had a higher risk of pregnancy-induced hypertension (high blood pressure) and certain bleeding disorders.

What Blood Type May Reveal About Cancer Risk

Blood type has also been linked to certain cancers, particularly stomach and pancreatic cancer. Multiple studies have found that people with type A blood appear to have a higher risk of stomach cancer than people with type O blood. 

Studies have also suggested that people with blood types A, B, and AB may be more prone to contracting pancreatic cancer compared with type O individuals. In people with type A, B, or AB blood, the ABO gene (which determines your blood type) may also increase the risk of lung, breast, cervical, and colorectal cancer.

Researchers still don’t understand the mechanisms behind these associations, although an interaction with the bacterium that causes ulcers — H. pylori — may be to blame for the heightened risk of stomach cancer.

Your Gut and Your Blood Type

Curiously, type O blood — often associated with lower cardiovascular risk — has been connected to higher rates of peptic ulcers. Researchers have long observed that people with type O blood appear more susceptible to these painful sores, which form in the stomach or upper small intestine. 

Some scientists believe this may relate to how the bodies of people with type O blood handle infections of H. pylori. One meta-analysis — a study of studies — found that people with type O blood were the most likely to be infected with H. pylori. Certain blood types have also been linked with gallstones and other digestive conditions, though the evidence is less consistent.

Blood Types May Handle Infections Differently

Blood type may also influence our vulnerability to infections. For example, some evidence suggests type O blood may provide protection against severe malaria. One study among children in Mali found type O blood was associated with a 66% reduction in the odds of developing severe malaria compared with non-O blood. In this case, scientists think they may have found the mechanism: A protein secreted by the malaria parasite seems to have a harder time sticking to type O blood cells.

Meanwhile, other blood types may affect susceptibility to illnesses such as cholera, norovirus, and certain bacterial infections. Interestingly, researchers have found that certain blood groups are more common in regions where specific diseases are more common. Scientists believe these patterns may have evolved over thousands of years as infectious diseases shaped human populations. In other words, blood-group diversity may partly reflect humanity’s long evolutionary battle with pathogens.

A Possible Link to Brain Health

A smaller but intriguing body of research suggests blood type could even influence brain health. Some studies have found that people with AB blood may have a higher likelihood of cognitive impairment later in life. Researchers have speculated that clotting factors and cardiovascular health could partly explain the connection, since blood flow problems can affect the brain over time.

Blood Type and Kidney Stones

One of the more unexpected discoveries involves kidney stones. The large Swedish study examining health data from more than 5 million people found that type B appeared to have a lower risk of developing kidney stones compared with people who had type O. Researchers described this as a newly identified association and noted that the underlying mechanism remains unclear. It’s a good example of how scientists are still uncovering unexpected ways blood type may intersect with health.

Blood Type Is Only Part of the Story

Despite all these associations, scientists still have many unanswered questions. Researchers are still trying to determine exactly how blood-group molecules affect inflammation, clotting, bacteria, immune activity, and other biological systems. Some findings have also been inconsistent across studies and populations. 

It’s important to note that your blood type doesn’t determine your destiny. Experts say blood type should be viewed as one small piece of a much larger health puzzle, and that lifestyle factors — such as diet, exercise, sleep, and smoking habits — likely have a much greater impact on overall health than whether you’re type A, B, AB, or O. For most of us, the basics of taking care of yourself still matter most.

It’s 2 a.m., you’re sweating through the sheets, and every part of your body aches. You’ve got a fever, and you’re burning up from the inside out. So you fumble for a pill bottle, struggle a moment with the childproof cap,  and finally swallow an over-the-counter capsule. Maybe two. An hour later, you feel noticeably better. But what exactly happened? How can such a simple remedy convince your body to stop running hot?

The answer has to do with the fact that a fever isn’t your body overheating uncontrollably. It’s more like your internal thermostat is being deliberately turned up. Fever-reducing drugs such as aspirin, acetaminophen, and ibuprofen don’t cool you down directly, as if they were handy little chemical fans, but they do help reset your body’s thermostat. 

It’s a simple solution to a straightforward problem, but the way these medicines work is a different story — one whose ending still needs to be written.

The Heat Is Coming From Inside the House

Normally, your body works hard to maintain a relatively stable internal temperature that keeps you cool and comfortable. Deep in a brain region called the hypothalamus lies a kind of master thermostat that you can thank for achieving this homeostasis. It’s vital to systems that keep you alive, including your digestive and respiratory systems.

Think of your body as a computer or a car engine, which, if they run too hot, stop working correctly. They may even shut down completely. It’s important to keep your body temp stabilized to keep everything functioning properly.

This built-in defense mechanism creates a hostile environment for foreign viruses and bacteria — if an infection can’t take the heat, it needs to get out of the kitchen. Plus, higher temps help certain immune cells work more effectively.

That’s why fevers often begin with chills. As your temperature rises, you may shiver, pull on extra blankets, or feel cold. Your body is trying to reach the new target temperature dictated by that internal thermostat. Chills happen as your blood vessels constrict to conserve heat while your muscles contract to generate more of it, all in an effort to make you warmer.

Common fever reducers are used to interfere with that process. There are a few different kinds available over the counter, including acetaminophen (Tylenol) and non-steroidal anti-inflammatory drugs (NSAIDs). NSAIDs come in various types, including ibuprofen (Advil), naproxen (Aleve), and aspirin.

Scientists are still teasing out how some medicines work to bring down a fever. But it’s now thought that a hormone-like substance called prostaglandin E2 plays a key role. During an infection, levels of this organic compound rise, telling your master thermostat to get hotter. By reducing the effects of prostaglandin E2, certain drugs help guide the hypothalamus to lower your internal temperature back toward normal.

A fever isn’t really the enemy, since it’s meant to aid your immune system (though if the fever runs too high or lasts too long, that’s a different story). Once medicine turns down your thermostat, your body switches strategies. Blood vessels near the skin widen, allowing heat to escape, and sweating helps cool the body further.

So the next time your fever reducer of choice helps you get through a long, difficult night, remember that it isn’t acting like an internal air conditioner. Instead, it’s turning off the furnace, and allowing your body to return to its usual setting.