Have you recently had a cold, flu, RSV, or COVID-19 and your cough just doesn’t go away? You’re not the only one. Symptoms may persist for several weeks after our bodies have eliminated the virus.
Michael Shiloh, an infectious disease research physician at UT Southwestern Medical Center in Texas, said patients with a cough often report the condition for up to eight weeks before coming to his office. “We can’t actually detect the virus in these people anymore, but they still have a cough,” he said.
In late 2023, the United States experienced an increase in influenza, respiratory syncytial virus, and COVID-19 cases that continued into 2024. Although positive test results for these illnesses have stabilized or decreased nationwide in recent weeks, the number of people seeking medical care and rates of respiratory illnesses remain high in much of the United States.
Scientists still don’t know why healthy people develop this persistent cough. But research into how infections interfere with airway nerves is revealing new clues.
Lorcan McGarvey, a pulmonologist and researcher at Queen’s University Belfast (Northern Ireland), says coughing is an important reflex that protects the respiratory tract from dangers such as harmful gases, water or poorly swallowed food particles.
This reflex is triggered by nerves that reach the airway. These nerves are packed with receptor proteins that respond to everything from cold air to capsaicin, the chemical that makes chili peppers spicy. When an irritant activates these receptors, the nerve sends a signal to the brain via the vagus nerve, and we feel the urge to cough.
From there, the brain sends commands to the airways to cough or not cough. This bypass of the brain is why some types of coughs can have a degree of conscious control.
Scientists know that many different stimuli can trigger a cough, but they still don’t agree on the specific biological mechanisms that trigger a cough when we’re sick, let alone in the weeks after.
While coughing appears to clear mucus from the throat, it’s also possible that the virus triggers reflexes that help spread itself. Many infections are accompanied by a dry cough that produces no phlegm at all. If we end up coughing to clear our airways during an infection, that still doesn’t explain what exactly our nerves feel during an infection-induced cough.
“We don’t know,” said Thomas Taylor-Clark, an electrophysiologist at the University of South Florida. “But what we can say is that we know a few things, and one of them is that viral infections cause inflammation.”
Research shows that when we are sick, inflammation triggers sensitivity in the nerves in our airways, causing them to overreact. One hypothesis is that persistent coughing after infection occurs when nerves remain highly sensitive after the illness is over.
“Breathing deeply, talking on the phone, laughing, going out for a breath of cold air…these things by themselves are not harmful or harmful. But, at least temporarily, they can cause coughing” even if we no longer get sick from coughing, McGarvey said.
As early as 1990, researchers observed that infection with a flu-like virus increased the sensitivity of nerves in the airways of guinea pigs, causing them to cough just like humans. Sick guinea pigs cough more than healthy guinea pigs when exposed to irritants such as capsaicin, which has also been observed in humans.
Now, scientists have linked this hypersensitivity to specific inflammatory chemicals and receptors in the nerves of the airways.
In 2016, scientists discovered that infection with a flu-like virus causes certain nerves in the airways to produce extra copies of a receptor protein called TRPV1 that responds to capsaicin and other irritants. Other studies, many of them conducted in laboratory-grown guinea pig or human cells, also revealed increases in copy number of the receptor protein TRPV1 and other receptors during respiratory virus infection.
A 2017 study showed that the copy number of TRPV1 and another receptor increased in lab-grown human cells even when the virus was “killed” by ultraviolet light.
A post-infectious cough usually goes away within two to three weeks. However, if the cough persists for more than eight weeks, Shiloh and McGarvey recommend seeing a doctor. Additionally, they recommend seeing a doctor first if the cough is accompanied by other symptoms, such as fever, difficulty breathing, blood in the sputum, or weight loss.
Lukasz Antoniewicz, a pulmonologist at the Chronic Cough Unit at the Medical University of Vienna, Austria, said it is normal to have a cough for several weeks after illness. However, he said that although the official limit for chronic cough is eight weeks, in reality relief may not occur for a long time.
“I have young patients, about 30 years old, who have broken ribs just from coughing, and they don’t have any underlying bone disease. So it’s a very, very strong reflex. And it hurts,” Antonovich said . “When you have a cough for a month, I would say that’s probably a good reason to see a doctor.”