Several theories try to explain why we cough when we have the flu or common cold.
Coughing is a normal reflex to clear the airway of small particles, microorganisms, mucus, or food or drinks accidentally going down "the wrong pipe." But when we are plagued with a pesky cold or flu infection, cough takes on a new dimension.
What often starts as a tickle in the throat at the first sign of an upper respiratory tract infection can easily turn into a full-blown dry cough, most likely followed by a hacking, mucus-producing cough that can be very uncomfortable.
Why do we cough when we have a flu or cold infection? We examine what research has revealed about the mechanisms behind cough and who benefits: the virus, or us?
The study of coughing
In an article in the journal BMJ Open Respiratory Research, Prof. Alyn Morice — head of respiratory medicine at the Centre for Cardiovascular and Metabolic Research at Hull York Medical School in the United Kingdom — explains how difficult it is to study coughing.
Humans naturally infected with rhinovirus, which is the cause of the common cold, or the flu virus, which causes influenza, do not make good study subjects as their symptoms vary greatly from person to person.
Instead, cells from our airways and guinea pigs are the models of choice when it comes to studying cough.
While scientists can't agree on one definitive mechanism that elicits the bellowing cough that you might be listening to while reading this article, there are several theories, says Prof. Morice.
Inflammation, cell damage, and mucus
Cold and flu viruses attack our bodies through our nasal lining. In response, our immune systems try to fight back by opening a pandora's box of inflammatory chemicals.
Prof. Morice goes on to explain that human rhinovirus infection is known to lead to high levels of pro-inflammatory molecules, such as bradykinin — also implicated in causing sore throat — tachykinins, calcitonin gene-related peptide, and leukotrienes.
While these play important roles in the battle against the common cold, some scientists point the finger squarely at these molecules as the reason for our cough.
Damage to the cell lining in our airways is common in influenza infections and may be why we tend to experience more severe coughing fits with flu than with the common cold, others think.
Mucus production is also in the mix of factors to lay the blame on, as it is known to stimulate nerve receptors. This leads us to the crux of cough: nerve endings in our airways.
The vagus nerve and cough
Cough is a neuronal reflex, mediated by the vagus nerve. The nerve endings responsible sit at the level of our larynx or lower in our airways, and cough signals must penetrate this far into the airway to cause symptoms.
That's why colds confined to the nose and head — so-called head colds — don't tend to cause cough, explains Prof. Ron Eccles, who was previously director of the Common Cold Centre in the School of Biosciences at the University of Cardiff in the U.K.
So, it seems that cough comes to no advantage to ourselves.
"The most common form of cough is caused by upper respiratory tract infections and has no benefit to the host."
Prof. Alyn Morice
The beneficiaries are the viruses at the root of our infections in the first place. They hijack coughs to spread their nasty offspring among the general population, while we are left paying the price with yet another coughing fit.
Coughing is temporary
The good news is that most coughs clear up within 3 weeks, although some — known as postviral or postinfectious cough — can last for up to 8 weeks.
Unfortunately, there is little evidence to suggest that cough medicines are effective in easing your cough.