The odds are pretty good that you know someone with diabetes. Affecting more than 30 million Americans, it's an incredibly common—and commonly misunderstood—condition.

The word diabetes comes from the Greek for "siphon"—a reference to the frequent and copious urination the condition can cause. The term was coined in the first century by ancient physician Aretaeus the Cappadocian, who vividly (and inaccurately) described the theory that "great masses of flesh are liquefied into urine."

Today we know a bit more about this illness, what causes it, and the forms it can take.

Diabetes is ultimately a hormone problem. The hormone in question is insulin, which helps the body convert glucose (sugar) into energy. Your pancreas releases a little dose of insulin into your bloodstream when you eat. The insulin tells certain cells to gobble up the glucose you've just added. The cells take in the sugar and put it to work.

Or at least that's how it's supposed to go. If you've got diabetes, the situation looks a little different.

Like rheumatoid arthritis or celiac disease, type 1 diabetes is the result of a person being attacked by their own immune system. In rheumatoid arthritis, the issue manifests in the joints; in celiac disease, it occurs in the gut; and in type 1 diabetes, it's the insulin-producing cells in the pancreas that are targeted by the immune system.

Little fluctuations in blood sugar that would breeze right through a healthy system can wreak havoc in the body of someone with type 1. People with type 1 must keep a very close eye on their glucose levels and take supplemental insulin, in shots or through a pen, port, pump, or inhaler, as blood sugar that goes too low or too high can cause serious complications and even death.

Type 2 diabetes is caused by an obstacle at the other end of the road. Someone with type 2 diabetes typically may have enough insulin to function, at least to start; the problem is that their body can't process it. Unused glucose builds up in the bloodstream and the body begins to need more and more insulin to see any effect.

Type 2 used to be known as adult-onset diabetes and type 1 as juvenile diabetes, but both kids and adults can and do develop both types. And while being overweight or obese does increase a person's risk of developing diabetes, thin people get it too. To complicate matters even further, researchers in Finland and Sweden recently identified five subgroups of diabetes, each with its own unique characteristics and risks for complications. Knowing which subgroup people fall into may improve treatment in the future.

And while we're myth-busting: The idea that diabetes is the product of eating too much sugar is a gross oversimplification. How you eat affects your body, of course, and a low-carb diet can help keep blood sugar in check, but diabetes can be caused by a lot of different factors, including genetics, medications, and other health conditions. (If you're on insulin, talk to a doctor before starting a low-carb diet, as low blood glucose levels can result if not done carefully.)

There's no common cure for diabetes—at least not yet. An artificial pancreas and other treatments for the immune system and pancreas cells are all in the works. In the meantime, both types can usually be managed with medication, diet changes, exercise, and a lot of doctor visits.