Diabetes develops through several distinct pathways depending on the type. In type 1 diabetes, the immune system destroys the cells that make insulin. In type 2, which accounts for roughly 90% of all cases, the body gradually loses its ability to use insulin effectively. Other forms can be triggered by medications, pancreatic damage, or pregnancy. More than 800 million adults worldwide now live with diabetes, with global prevalence doubling from 7% to 14% between 1990 and 2022.
How Type 1 Diabetes Develops
Type 1 diabetes is an autoimmune disease. The immune system mistakenly attacks the insulin-producing beta cells in the pancreas, eventually destroying enough of them that the body can no longer regulate blood sugar on its own. This process can unfold over months or years before symptoms appear. In fact, researchers have observed a sharp rise in blood sugar after meals as early as two months before the immune attack is even detectable through standard antibody tests.
The destruction itself is surprisingly subtle at the cellular level. Fewer than 10% of the insulin-producing cell clusters (called islets) become infiltrated by immune cells, and it takes only about 15 immune cells per islet to cause damage. That’s just twice the number found in a healthy pancreas. But over time, this low-level assault is enough to wipe out insulin production entirely.
Genetics set the stage, but something in the environment usually pulls the trigger. Certain viral infections are strongly linked to the onset of type 1 diabetes, particularly enteroviruses like Coxsackievirus. These viruses produce proteins that closely resemble proteins on the surface of beta cells. The immune system, primed to fight the virus, can mistake beta cells for the invader and begin attacking them. Rotavirus and cytomegalovirus have also been implicated. The TEDDY study, a large international childhood study, found that gastrointestinal infections before age one were associated with an increased risk of developing the autoimmune markers that precede type 1 diabetes.
People carrying certain gene variants in the HLA system (a set of genes that govern immune responses) appear most vulnerable to this molecular mimicry. Early introduction of gluten in infancy has also been linked to increased risk, though the connection is less established than with viral triggers. Type 1 diabetes most commonly appears in childhood or adolescence, but it can develop at any age.
How Type 2 Diabetes Develops
Type 2 diabetes follows a slower, more gradual path. It begins with insulin resistance: your cells stop responding normally to insulin, so your pancreas has to produce more and more of it to keep blood sugar in check. For a while, this compensation works. But eventually the pancreas can’t keep up, insulin production falls behind demand, and blood sugar starts to rise.
Excess body fat, particularly fat stored around the organs in the abdomen (visceral fat), is the primary driver of insulin resistance. This type of fat tissue releases inflammatory signals that interfere with insulin’s ability to move sugar from the blood into cells. The more visceral fat you carry, the louder those signals become and the harder your pancreas has to work.
Before full diabetes sets in, most people pass through a stage called prediabetes, where blood sugar is elevated but not yet in the diabetic range. Without changes to diet, activity, or weight, many people with prediabetes develop type 2 diabetes within five years. The transition isn’t inevitable, though. Weight loss has a powerful effect on reversing the process, and the more weight lost, the better the odds. A large review of clinical trials found that people who lost less than 10% of their body weight had less than a 1% chance of complete remission at one year. Those who lost 20 to 29% had about a 50% remission rate, and those who lost 30% or more saw remission rates near 80%. Each percentage point of weight loss increased the probability of remission by roughly two percentage points.
The Role of Genetics
Family history is one of the strongest predictors of type 2 diabetes. If one of your parents has the disease, your lifetime risk of developing it is around 40%. If your mother is the affected parent, the risk is somewhat higher than if your father is. If both parents have type 2 diabetes, the risk climbs to approximately 70%. These numbers reflect shared genetics, but families also tend to share eating patterns, activity levels, and other habits that compound the genetic risk.
Diabetes Caused by Other Medical Conditions
Some people develop diabetes not through autoimmunity or insulin resistance, but because of direct damage to the pancreas. Chronic pancreatitis, which is often caused by gallstones or heavy alcohol use, can scar pancreatic tissue and reduce insulin production over time. Pancreatic cancer or surgery that removes part of the pancreas can have the same effect. When diabetes results from this kind of structural damage, it’s sometimes classified as type 3c diabetes.
Cystic fibrosis is another condition that raises diabetes risk, because thick mucus can block and damage the pancreas over the course of the disease. Hemochromatosis, a condition where the body absorbs too much iron, also increases the risk. Excess iron gets stored in the liver and pancreas, where it impairs insulin production directly.
Medications That Can Trigger Diabetes
Several widely prescribed medications raise blood sugar enough to cause diabetes in some people. Corticosteroids (commonly called steroids) are among the most significant. They block insulin’s action and cause the liver to release extra glucose into the bloodstream. Oral steroids cause a slower, sustained rise in blood sugar, while steroid injections can spike levels quickly for three to ten days.
Atypical antipsychotic medications, used to treat schizophrenia and other psychiatric conditions, can increase hunger, promote weight gain, and directly reduce insulin production. High-dose statins, the cholesterol-lowering drugs taken by millions of people, carry a small but real increase in diabetes risk. Beta blockers, prescribed for high blood pressure and certain heart conditions, have also been linked to elevated blood sugar.
How Diabetes Is Diagnosed
Diabetes is identified through blood sugar measurements, and there are three main tests used. A fasting blood glucose test measures sugar after at least eight hours without eating. Normal is below 100 mg/dL, prediabetes falls between 100 and 125, and 126 or higher indicates diabetes. The A1C test reflects your average blood sugar over the previous two to three months. Normal is below 5.7%, prediabetes is 5.7 to 6.4%, and 6.5% or higher means diabetes. An oral glucose tolerance test measures blood sugar two hours after drinking a sugary solution: below 140 is normal, 140 to 199 is prediabetes, and 200 or above is diabetes.
Roughly 59% of all adults with diabetes globally, nearly 450 million people, are untreated. Many don’t know they have it. Type 2 diabetes in particular can develop silently for years, causing damage to blood vessels, nerves, and organs before symptoms like increased thirst, frequent urination, or blurred vision become obvious enough to prompt a test.