Cold and flu virus symptoms often mild in non-smokers can seriously hit smokers, according to a study by Yale School of Medicine.
The study identified the mechanism by which viruses and cigarette smoke interact to increase lung inflammation and damage.
Until recently, scientists were unable to explain why smokers had more exaggerated responses to viral infections. Smokers have been more likely than non-smokers to die during previous influenza epidemics and are more prone to chronic obstructive pulmonary disease (COPD).
Furthermore, children who are exposed to second-hand smoke have more severe responses when infected with respiratory synctial virus.
The prevailing view has been that cigarette smoke decreases anti-viral responses. But the Yale researchers-co-authors Jack A. Elias and Min-Jong Kang, found the opposite to be true.
Their experiments showed that the immune systems of mice exposed to as little as two-cigarette smoke a day for two weeks over reacted when they were also exposed to a mimic of the flu virus. The mice's immune systems cleared the virus normally but the exaggerated inflammation caused increased levels of tissue damage.
"The anti-viral responses in the cigarette smoke exposed mice were not only not defective, but were hyperactive," said Elias. "These findings suggest that smokers do not get in trouble because they can't clear or fight off the virus; they get in trouble because they over-react to it.
"It's like smokers are using the equivalent of a sledge hammer, rather than a fly swatter, to get rid of a fly," said Elias.
These findings have been published in the Journal of Clinical Investigation.