Diabetes develops when your body either stops making insulin, stops responding to it properly, or both. But the causes vary dramatically depending on the type. Type 1 diabetes is driven by an immune system attack on the pancreas. Type 2 is rooted in how your body handles fat and inflammation over time. And several less common forms have their own distinct triggers, from pregnancy hormones to genetic mutations to physical damage to the pancreas.
Type 1: The Immune System Attacks the Pancreas
Type 1 diabetes is an autoimmune disease. Your immune system, which normally fights infections, turns against the insulin-producing beta cells in the pancreas and destroys them. Without these cells, your body can’t make insulin at all.
The attack is carried out primarily by T cells, a type of white blood cell. Both CD4 and CD8 T cells are involved, and together they’re highly effective at killing beta cells. Macrophages, another type of immune cell, are the first to infiltrate the pancreas during the disease process. Once there, they release inflammatory signals that recruit more immune cells and amplify the damage. The beta cells themselves become part of the problem: under sustained inflammatory stress, they produce nitric oxide, which damages their own DNA and disrupts their ability to generate energy. Over time, enough beta cells die that insulin production drops to near zero.
What triggers the immune system to attack in the first place isn’t fully pinned down, but viral infections are a strong contributor. Enteroviruses are the most studied link. A meta-analysis found a significant association between enterovirus infections and the development of pancreatic autoimmunity, particularly when a child experiences repeated or prolonged infections. Rotaviruses, cytomegalovirus, and human herpesvirus 6 have also been implicated. One mechanism is molecular mimicry: viral proteins resemble proteins on beta cells closely enough that the immune system confuses the two. The COVID-19 pandemic offered a natural experiment. A meta-analysis of over 38,000 newly diagnosed cases in children found that type 1 diabetes incidence was 14% higher in the first year after the pandemic began and 27% higher in the second year.
Genetics matter too, but they set the stage rather than pull the trigger. Having certain immune system gene variants increases susceptibility, but most people with those genes never develop type 1 diabetes. It takes an environmental trigger, likely a viral infection in someone who is genetically predisposed, to set the autoimmune process in motion.
Type 2: Insulin Resistance and Chronic Inflammation
Type 2 diabetes accounts for roughly 90% of all diabetes cases worldwide. It develops when your cells gradually lose their ability to respond to insulin, a condition called insulin resistance, and your pancreas can’t compensate by making enough extra insulin to keep blood sugar in check.
Excess body fat, particularly around the organs in your abdomen, is the single biggest driver. Fat tissue isn’t passive storage. When fat cells grow too large (a process called adipocyte hypertrophy), they become inflamed. Immune cells called macrophages infiltrate the fat tissue, and inflammatory signals spill into the bloodstream. This low-grade, bodywide inflammation directly impairs how insulin works in fat tissue, the liver, and skeletal muscle. Research in animal models has shown that reducing this fat tissue inflammation, even without changing body weight, improves insulin sensitivity and glucose tolerance. The inflammation is the direct link between excess fat and insulin resistance.
Physical inactivity compounds the problem. Skeletal muscle is the largest consumer of blood sugar in your body, and regular movement keeps muscle cells sensitive to insulin’s signal to absorb glucose. When you’re sedentary, that sensitivity drops. Combined with excess calorie intake, inactivity creates a feedback loop: more fat accumulation, more inflammation, worsening insulin resistance, higher blood sugar, and eventually a pancreas that can no longer keep up.
Your gut bacteria also play a role. In people with type 2 diabetes, the balance of gut microbes shifts in ways that increase intestinal permeability. This allows bacterial toxins to leak into the bloodstream, fueling systemic inflammation and worsening metabolism. The gut also produces fewer beneficial compounds like short-chain fatty acids, which normally help regulate blood sugar and protect the gut lining.
Genetics influence type 2 diabetes risk as well. Having a parent or sibling with the condition roughly doubles or triples your risk. Certain ethnic groups, including South Asian, Black, Hispanic, and Indigenous populations, face higher rates even after accounting for lifestyle factors. But unlike type 1, type 2 diabetes is heavily influenced by modifiable factors: body weight, diet, and activity level.
Gestational Diabetes: Hormones That Block Insulin
During pregnancy, the placenta produces hormones that help the baby develop. Three of these, estrogen, cortisol, and human placental lactogen, also partially block insulin’s effect on the mother’s cells. All pregnant women experience some degree of increased insulin resistance in the second and third trimesters. Gestational diabetes develops when the mother’s pancreas can’t produce enough additional insulin to overcome that resistance.
Risk factors include being overweight before pregnancy, being over 25, having a family history of type 2 diabetes, and having had gestational diabetes in a previous pregnancy. The condition usually resolves after delivery when placental hormones leave the system, but it signals an underlying vulnerability. Women who develop gestational diabetes have a significantly higher lifetime risk of developing type 2 diabetes later.
Pancreatic Damage and Disease
Any condition that physically damages the pancreas can cause diabetes by destroying the cells that make insulin. Chronic pancreatitis is the most common example. As the pancreas becomes progressively scarred and fibrotic, blood supply to the insulin-producing islets is cut off, leading to a steady loss of functional beta cells. In some cases, the beta cells don’t die outright but instead transform into a different cell type that can no longer produce insulin.
Acute pancreatitis can also trigger diabetes if the inflammation is severe enough to cause significant tissue death. The more pancreatic tissue destroyed by necrosis, the fewer functioning beta cells remain.
Cystic fibrosis causes a different kind of pancreatic destruction. Defective chloride channels produce thick, sticky mucus that blocks the pancreatic ducts, leading to premature activation of digestive enzymes inside the organ. Over time, the pancreas becomes extensively scarred and infiltrated with fat. Patients with cystic fibrosis-related diabetes typically lose around 50% of their islet cells, enough to significantly impair insulin production.
Medications That Raise Blood Sugar
Corticosteroids, commonly prescribed for conditions like asthma, arthritis, and autoimmune diseases, are the most well-known drug-related cause of elevated blood sugar. They work against insulin on multiple fronts: they increase fat storage around the organs, stimulate the breakdown of muscle and fat tissue, and release fatty acids into the blood that interfere with insulin signaling. They also directly harm beta cells by triggering stress responses that can lead to cell death. The result is both increased insulin resistance and reduced insulin production at the same time.
People with pre-existing insulin resistance or prediabetes are most vulnerable to corticosteroid-induced blood sugar problems. For some, stopping the medication resolves the issue. For others, it can unmask a lasting tendency toward diabetes.
Genetic Forms: MODY and LADA
Not all diabetes fits neatly into the type 1 or type 2 categories. Maturity-onset diabetes of the young (MODY) is caused by a mutation in a single gene and typically appears before age 25. It’s inherited in a dominant pattern, meaning if a parent carries the mutation, each child has a 50% chance of developing it. MODY doesn’t involve autoimmune destruction or insulin resistance. Instead, the genetic defect directly impairs how the pancreas senses blood sugar or secretes insulin.
Latent autoimmune diabetes in adults (LADA) is sometimes called type 1.5 because it shares features of both major types. Like type 1, it involves autoimmune destruction of beta cells, with detectable autoantibodies in the blood. But like type 2, it develops slowly, often in adults over 30, and may initially be managed without insulin. Some people with LADA also have insulin resistance, particularly those who are overweight. Because of its gradual onset, LADA is frequently misdiagnosed as type 2 diabetes, which can delay appropriate treatment.