It was once considered primarily a respiratory illness, but COVID-19 has consistently surprised doctors by the sheer number of body systems it can affect, including the brain and the heart. Now, researchers say, the coronavirus may even attack cells in the pancreas, causing diabetes.In the Sept. 2 issue of the journal Nature Metabolism, scientists reported on the case of a 19-year-old German man who developed coronavirus after a family vacation. He had no traditional symptoms of COVID-19, until he ended up in the hospital with a 26-pound weight loss inside a few weeks, frequent urination and pain in his left side. Doctors found his blood sugar level was more than three times the normal range. They suspected the patient had Type 1 diabetes (which is usually diagnosed in early childhood), but he tested negative for genetic variants and antibodies normally associated with Type 1. Read on, and to ensure your health and the health of others, don't miss these Sure Signs You've Already Had Coronavirus. Both diabetes and COVID can cause immune overreactionPeople develop diabetes when the pancreas stops producing enough insulin, an enzyme that ushers sugar from the blood into cells, where they can use it for energy. Excessive blood sugar can damage the walls of blood vessels, which can lead to heart disease, stroke, blindness and amputation.In the case of the 19-year-old German patient, researchers theorize that the coronavirus caused his immune system to attack beta cells in the pancreas, which process insulin. Those cells contain many neurotransmitters known as ACE2 receptors. Coincidentally, those receptors are where the spiked surface of the coronavirus attach to cells. (Type 1 diabetes is also believed to be an autoimmune disorder, in which the body attacks healthy beta cells and shuts them down.)RELATED: COVID Mistakes You Should Never MakeResearchers have reported other cases of COVID-19 triggering diabetes. Dr. Caroline Messer, an endocrinologist at Lenox Hill Hospital in New York City, told WebMD there has been an increase in autoimmune diabetes during the coronavirus pandemic and that the researchers' theory is a plausible explanation. "This could account for the uptick in antibody negative type 1 diabetes," she said. "It is important for practitioners to be aware of the possibility of insulin-dependent diabetes approximately four weeks after infection in spite of negative [type 1 diabetes] antibodies."But not all experts are convinced — some question whether this diabetes is Type 1, Type 2 (which traditionally has been reversible in many cases via diet and exercise) or a different category called sudden-onset diabetes. Some cases of coronavirus-related diabetes have resolved over time; Type 1 diabetes is generally not reversible. Diabetes symptoms to watch forAccording to the American Diabetes Association, symptoms of diabetes include extreme fatigue, excessive thirst or hunger, weight gain or weight loss and blurred vision. Talk to your healthcare provider if you experience any of those signs with or without a COVID-19 diagnosis.As for yourself, do everything you can to prevent getting—and spreading—COVID-19 in the first place: Mask, get tested if you think you have coronavirus, avoid crowds (and bars, and house parties), practice social distancing, only run essential errands, wash your hands regularly, disinfect frequently touched surfaces, and to get through this pandemic at your healthiest, don't miss these 35 Places You're Most Likely to Catch COVID.