Diarrhea can be a symptom of the flu, though it’s far less common than the hallmark signs like fever, cough, and body aches. A large meta-analysis found that roughly 24% to 31% of people with confirmed influenza A reported gastrointestinal symptoms, with the exact rate depending on the viral subtype. So while diarrhea isn’t the defining feature of influenza, it’s not unusual either.
How Common Diarrhea Is During the Flu
The flu is primarily a respiratory illness, so most people experience it as a combination of fever, chills, sore throat, muscle aches, and cough. Digestive symptoms sit further down the list but still show up in a meaningful number of cases. The H1N1 strain (the type behind the 2009 pandemic and still circulating seasonally) tends to cause more gut symptoms, with about 31% of confirmed cases reporting them. The H3N2 subtype comes in lower at around 24%.
Children are more likely than adults to develop vomiting and diarrhea during a flu infection. These symptoms can overlap with what parents often call the “stomach flu,” which creates confusion. In adults, digestive complaints during influenza are less frequent and typically milder, often taking a backseat to the respiratory and systemic symptoms that dominate the illness.
Why a Respiratory Virus Affects Your Gut
It seems counterintuitive that a virus you breathe in would cause diarrhea, but research from the CDC shows that influenza A viruses can spread from the respiratory tract to the gastrointestinal tract after initial infection. Animal studies in ferrets have confirmed that both seasonal and novel influenza strains travel to gut tissue after nasal infection. Lab studies have also shown that different influenza subtypes can multiply within human intestinal tissue, which helps explain why some people develop genuine digestive symptoms rather than just feeling nauseated from being sick.
This means the diarrhea isn’t always just a side effect of fever or medications. In some cases, the virus is actively present in the gut and contributing directly to inflammation there.
Flu Diarrhea vs. the “Stomach Flu”
The term “stomach flu” is misleading because it has nothing to do with influenza. What people call the stomach flu is actually viral gastroenteritis, most commonly caused by norovirus. The two illnesses look quite different in practice.
- Primary symptoms: Norovirus hits hardest in the gut, with diarrhea, vomiting, nausea, and stomach pain as the main features. Influenza hits hardest in the respiratory system and muscles, with fever, cough, congestion, and body aches leading the way.
- Onset: Norovirus symptoms appear 12 to 48 hours after exposure. Flu symptoms typically take one to four days.
- Duration: Norovirus usually resolves within one to three days. The flu lasts longer, often five to seven days for the worst symptoms, with fatigue lingering beyond that.
- Contagion: You can spread norovirus for two weeks or more after you feel better. Flu contagiousness drops off more quickly, usually within a week of symptom onset.
If diarrhea and vomiting are your dominant symptoms and you have little to no cough or respiratory involvement, norovirus or another stomach bug is far more likely than influenza.
Antiviral Medications Can Add to the Problem
Here’s a wrinkle worth knowing: the most commonly prescribed flu antiviral, oseltamivir (Tamiflu), lists diarrhea, nausea, and vomiting among its most common side effects. So if you start an antiviral and your diarrhea gets worse or appears for the first time, the medication itself may be contributing. This side effect is generally mild and tends to ease as your body adjusts. Severe or bloody diarrhea during antiviral treatment is rare but worth flagging to your doctor.
Dehydration Is the Real Risk
Diarrhea during the flu is usually self-limiting and resolves as the infection clears. The bigger concern is dehydration, especially when diarrhea stacks on top of fever (which increases fluid loss through sweating) and reduced appetite. Children, older adults, and people with chronic conditions are most vulnerable.
Signs that dehydration is becoming a problem include unusual sleepiness or irritability, dark urine or very infrequent urination, dry mouth, and dizziness when standing. For children especially, an inability to keep fluids down is a warning sign. The Mayo Clinic flags several thresholds that warrant a call to your doctor: diarrhea lasting 24 hours or more, confusion or unusual drowsiness, inability to keep fluids down, bloody or black stool, or a fever reaching 102°F or higher.
Small, frequent sips of water, broth, or an oral rehydration solution work better than trying to drink large amounts at once, which can trigger more nausea. Avoiding caffeine and sugary drinks helps, since both can worsen diarrhea.
How COVID-19 Compares
Both influenza and COVID-19 can cause diarrhea, but the pattern differs slightly. With the flu, diarrhea shows up more often in children than adults. With COVID-19, digestive symptoms including diarrhea can appear at any age and have been a more consistently reported feature of the illness across age groups. If you’re unsure which virus you’re dealing with, a rapid test is the most reliable way to tell them apart, since the symptom overlap between the two is significant.