Diagnosis
You wake up with cramping, urgency, and diarrhea. If you have ulcerative colitis (UC), your first thought is probably the same one every UC patient has had: is this a flare, or did I just catch something? The answer matters more than you might think. A UC flare may call for steroids or a biologic dose adjustment, while a gastrointestinal infection typically needs to run its course. Worse, treating a gut infection with steroids can actually make the infection harder to fight. So figuring out which one you're dealing with is worth the effort, even when the symptoms overlap.
The Symptoms That Overlap
UC flares and stomach bugs share a frustrating amount of common ground. Both can cause diarrhea, abdominal cramping, urgency, fatigue, and even fever. That overlap is why so many UC patients end up in a cycle of uncertainty every time their gut acts up. According to the Crohn's & Colitis Foundation, the early signs of an IBD flare often mimic acute infections like food poisoning or norovirus. And to complicate things further, a gut infection can actually trigger a UC flare, meaning you can sometimes have both happening at once.
How Onset and Timing Differ
One of the most reliable clues is how the symptoms arrive. Stomach bugs, whether viral or bacterial, tend to hit suddenly. You feel fine in the morning and terrible by the afternoon. A UC flare, on the other hand, more often builds gradually. You might notice a slight increase in urgency or looser stools over several days before things escalate. That said, severe UC flares can also come on fast, so onset alone is not a definitive answer.
Duration is another useful signal. Most stomach viruses resolve within three to seven days, and food poisoning often clears in 24 to 48 hours. If your symptoms persist beyond a week, or if they follow a slow-building pattern rather than a sudden spike, a flare becomes more likely. The Crohn's and Colitis UK guidelines suggest that going to the toilet more than five times in 24 hours, or experiencing bloody diarrhea for more than three days, are signs pointing toward a flare.
The Blood-in-Stool Question
For UC patients specifically, blood and mucus in the stool are among the strongest differentiators. While some bacterial infections can cause bloody stool, most common stomach bugs (norovirus, rotavirus, mild food poisoning) do not. If you see blood or mucus, especially if it follows the pattern you recognize from previous flares, that signal leans heavily toward UC activity rather than infection. The Mayo Clinic lists bloody diarrhea with mucus as a hallmark of active UC.
Vomiting can also be informative. Stomach bugs frequently cause nausea and vomiting, particularly early on. UC flares rarely do. If vomiting is a prominent symptom, an infection is more likely the culprit.
What Your Doctor Will Do
When you call your gastroenterologist with this question, they will likely order stool tests. A stool culture or gastrointestinal pathogen panel can identify bacterial or viral infections. Equally important is testing for Clostridioides difficile (C. diff), a bacterial infection that IBD patients are at higher risk for and that presents almost identically to a UC flare. The American College of Gastroenterology recommends that all IBD patients experiencing flare-like symptoms be screened for C. diff because the treatment approaches are so different.
Your doctor may also check fecal calprotectin, a marker of intestinal inflammation. Elevated calprotectin levels correlate strongly with UC disease activity and can help confirm whether your colon is inflamed beyond what a simple infection would cause. However, infections can also raise calprotectin, so this test works best in combination with stool cultures and your clinical history.
A Quick Decision Framework
While you wait to reach your doctor, ask yourself these questions:
Did symptoms appear suddenly or build over days? Sudden onset favors infection. A gradual build favors a flare.
Is there blood or mucus in your stool? Blood and mucus point toward UC activity.
Are you vomiting? Vomiting is more common with stomach bugs than UC flares.
None of these questions replace professional evaluation, and calling your GI team early is always the right move. But having clear answers ready when you call, especially about when symptoms started, whether you see blood, and how the pattern compares to your previous flares, helps your doctor make faster decisions about next steps.
Why Your Symptom History Matters Most
The single best tool for answering "flare or bug?" is knowing your own baseline. UC patients who track their daily symptoms can spot whether a bad day is a sudden departure from normal (suggesting infection) or part of a building pattern (suggesting a flare). That context is exactly what your gastroenterologist needs to hear when you call.
When you're unsure whether it's a flare or a bug, your symptom history is your best clue. Track daily with Aidy so you can spot whether symptoms are a sudden departure or a building pattern.