Undiagnosed diabetes does not directly cause fever, but it significantly raises your risk of infections that do. Persistently high blood sugar weakens your immune system in measurable ways, making you more vulnerable to urinary tract infections, respiratory infections, skin infections, and pneumonia, all of which can produce a fever. If you’re experiencing recurring or unexplained fevers alongside symptoms like unusual thirst, frequent urination, or fatigue, uncontrolled blood sugar could be the underlying driver.
Why High Blood Sugar Leads to More Infections
When blood sugar stays elevated, your white blood cells lose some of their ability to fight off pathogens. Normally, these cells use a molecule called NADPH to produce germ-killing compounds and coordinate an immune response. But in a high-glucose environment, your body diverts that same molecule toward processing the excess sugar, converting it into a sugar alcohol called sorbitol. This creates a competition for resources inside immune cells: the more glucose your body has to process, the less fuel is available for fighting infection.
The result is white blood cells that are slower to reach the site of an infection, less effective at killing bacteria and viruses once they arrive, and worse at forming the net-like traps they normally use to capture pathogens. High blood sugar also impairs the ability of blood vessels in the skin to dilate properly, which further limits the body’s ability to mount a localized immune response. None of this produces a fever on its own, but it creates the perfect conditions for infections to take hold, and those infections cause fever.
How Much the Risk Increases
The connection between blood sugar levels and infection risk is dose-dependent. A large cohort study found that people with diabetes were about 59% more likely to be hospitalized for an infection compared to people without diabetes. But when fasting blood sugar exceeded 200 mg/dL, a level common in undiagnosed or poorly managed diabetes, the risk of hospitalization for infection nearly tripled. The risk of dying from an infection at that blood sugar level was more than three times higher than in people without diabetes.
Even people in the prediabetic range (fasting glucose between 100 and 126 mg/dL) had a measurably higher infection risk compared to those with normal blood sugar. This matters because someone with undiagnosed diabetes often has no idea their blood sugar has been elevated for months or years, meaning their immune system has been compromised the entire time.
Infections That Commonly Cause Fever in Undiagnosed Diabetes
The CDC identifies several infections that occur more frequently in people with diabetes:
- Urinary tract infections, which can cause fever, pelvic pain, and burning during urination. High sugar in the urine creates a favorable environment for bacterial growth.
- Respiratory tract infections, including the common cold and bronchitis, which tend to linger longer and become more severe.
- Pneumonia, a potentially serious lung infection that almost always produces fever.
- Influenza, which hits harder and lasts longer when blood sugar is elevated.
- Skin infections, including bacterial infections and fungal infections like yeast, which can cause localized warmth and sometimes systemic fever.
When your body fights any of these infections, the immune response itself releases hormones that temporarily push blood sugar even higher. This creates a feedback loop: infection worsens blood sugar control, and worsened blood sugar control makes the infection harder to clear. For someone whose diabetes is undiagnosed, this cycle can spiral quickly because they aren’t taking any steps to manage their glucose.
Temperature Regulation Problems
Diabetes can also disrupt your body’s ability to regulate its own temperature, which adds another layer of complexity. Long-term high blood sugar damages the small nerve fibers that control sweating. In one study of diabetes patients with signs of nerve damage, 94% had measurable sweating abnormalities. About 16% had lost the ability to sweat across their entire body, and the severity of sweat loss correlated directly with the severity of nerve damage.
When your body can’t sweat properly, it loses one of its primary cooling mechanisms. This can make you more vulnerable to overheating in warm environments and can make any fever feel more extreme, since your body has fewer tools to bring its temperature back down. In type 2 diabetes specifically, impaired blood vessel dilation in the skin compounds this problem.
Symptoms That Overlap With a Simple Fever
Part of the reason undiagnosed diabetes goes undetected is that some of its symptoms look like a routine illness. Fatigue, headache, and general malaise are common in both a viral fever and in hyperglycemia. If you’ve been attributing these feelings to “just being sick a lot,” it’s worth paying attention to the symptoms that distinguish the two.
High blood sugar produces a specific pattern: increased thirst that doesn’t go away after drinking water, noticeably more frequent urination, blurred vision, and unexplained weight loss over weeks or months. Slow-healing cuts, recurring yeast infections, and skin infections that keep coming back are also hallmarks. A standard viral illness resolves in a week or two. If your fevers keep returning, or you notice these additional symptoms between bouts of illness, persistently elevated blood sugar could be the common thread.
When Fever Signals Something More Dangerous
In rare cases, uncontrolled diabetes can lead to two life-threatening emergencies that may involve fever. Diabetic ketoacidosis occurs when the body runs so low on usable insulin that it starts breaking down fat for fuel, producing toxic acids. Symptoms include nausea, vomiting, abdominal pain, a fruity smell on the breath, rapid breathing, and confusion. A low-grade fever can accompany this, often because an underlying infection triggered the crisis.
Hyperglycemic hyperosmolar syndrome is another emergency, more common in type 2 diabetes, where blood sugar climbs extremely high and the body becomes severely dehydrated. Early signs include dry mouth, a weak and rapid pulse, and a low-grade fever. In children, headache, nausea, and vomiting are more prominent. Both conditions can progress to seizures and loss of consciousness.
If you or someone near you has a fever combined with confusion, fruity-smelling breath, rapid breathing, chest pain, difficulty staying conscious, or weakness on one side of the body, that combination points to a medical emergency rather than a routine infection.