Canker sores are not necessarily a sign that you're getting sick with an illness. They are small, painful ulcers that develop inside the mouth and have multiple possible causes that don't involve infection. Common triggers include minor mouth injuries from dental work, aggressive brushing, accidentally cheek biting, or sharp foods; irritation from acidic foods like citrus fruits or tomatoes; stress and lack of sleep; nutritional deficiencies in vitamin B12, zinc, folate, or iron; or certain mouth irritants like sodium lauryl sulfate in some toothpastes. Most canker sores heal on their own within one to two weeks without treatment. However, canker sores can occasionally be associated with systemic conditions worth noting: they may appear during or before viral infections like the common cold or flu, though they're not required for these illnesses; they can indicate nutritional deficiencies that warrant dietary changes; rarely, they're linked to autoimmune conditions or celiac disease; and they can be a side effect of certain medications. If you're experiencing canker sores alongside other symptoms like fever, fatigue, swollen lymph nodes, or widespread mouth ulcers, those additional symptoms suggest you may be getting sick. Similarly, if canker sores appear unusually frequently, are extremely large, or last longer than three weeks, it's worth discussing with a healthcare provider. For typical isolated canker sores, they're simply a common minor mouth condition rather than an indicator of illness.