Let’s get right to the point: Have you ever had a cold and noticed that your poop seems to be acting weird? It turns out that when you have a cold, your poops are more frequent and more severe, thanks to your germs messing with your gut. Poop can actually tell you a lot about your health, since the body's waste-producing system is greatly affected by viruses, diet, and other factors. The gastrointestinal system and colon do reflect what's going on in the rest of the body, so don't be surprised if you notice something strange in your toilet during a winter illness.
Colds and flu can affect your stool in a number of ways. One is that the body produces mucus, although this is actually quite rare. When you're fighting a cold or flu, your respiratory system tends to go into overdrive, producing mucus phlegm as part of its protective healing process. Sometimes mucus can also appear in your stool, but this is not considered normal.
More commonly diarrhea occurs. This is considered a sign that you have a serious viral infection and not just a mild cold; a stuffy nose doesn't usually accompany diarrhea, but the flu certainly does. Some strains of influenza show early signs of diarrhea and vomiting.
Tests on mice in 2014 found that the flu virus can cause changes in the gut microbiome, the careful balance of bacteria and other substances in the gut that help us digest food and prevent disease. These changes appeared to "disrupt" the mice's intestinal balance, causing symptoms such as nausea and diarrhea. It's not entirely clear if this is exactly what happens in humans, but it's definitely a pretty viable hypothesis.
Even if you don't have the flu, though, a cold can change the composition of your stool because of one major factor: dehydration. Colds (including fever or sweating) can cause dehydration because the body loses water, which can lead to constipation. When you are dehydrated, your colon uses water from waste to meet its hydration needs, causing your stools to be hard and dry. This is another reason to drink more fluids when you have a cold.
There's another reason why the composition of your stool changes when you have a cold or flu, but it's not caused by the illness itself. Medications that help you cope with cold and flu symptoms often have side effects that cause problems with the body's waste disposal system. Pseudoephedrine, a common ingredient in cold medicines, can cause diarrhea, while pain relievers without steroids (called NSAIDs) can cause constipation. This can be especially difficult for people who already have gastrointestinal conditions like irritable bowel syndrome, as a cold or flu can cause their symptoms to flare up.
If you do come down with a cold or flu, there are certain things you should be aware of in your stool. There should be no blood in it, and you should talk to your doctor if constipation or diarrhea lasts for a long time and doesn't resolve on its own. Make sure you stay hydrated, watch carefully for the side effects of any medications you may be taking to help yourself feel better, and see your doctor if your stools don't look normal after a few days.
