Smoking on top of drinking alcohol leaves the body wide open to a bug that causes pneumonia, say scientists.
Smoking and drinking affects the body's defences against infections
They say they have shown for the first time that alcohol impairs an important defence barrier to lung infections, and that smoking intensifies this.
Rats exposed to alcohol and smoke lost movement of cilia - tiny hairs lining the airways that beat rhythmically to waft bugs up and out of the lungs.
The authors' paper in Alcoholism said the same was probably true in humans.
Researcher Professor Martha Gentry-Nielsen, of Creighton University School of Medicine, said there were several reasons why alcoholics were more susceptible to an infection called streptococcus pneumoniae, which can cause pneumonia.
"They have a decreased gag reflex," she said. This means it is easier for mucus from the nose and fluid from the gut, such as vomit, to go down into the lung, particularly when they lose consciousness, she said.
"Host defences within the lungs of alcoholics are also compromised, so they are not well equipped to handle the infection once the organisms reach the lungs."
She said that smoking compounded the problem.
"Smokers are much more likely to be colonised with the organism in their mouths and nasopharynx - the upper part of the nose - than non-smokers.
"Smoking also injures cilia and alters the efficiency of their beating so that bacteria entering the trachea have an increased likelihood of making their way into the lungs."
In the study, 64 rats were exposed twice-daily to either smoke generated from 30 cigarettes or smoke-free air for 12 weeks.
During the last five weeks, the rats were fed liquid diets that contained no, small, moderate or large amounts of alcohol and then infected with streptococcus pneumoniae via the nose.
The rats exposed to heavy alcohol and smoke were the most likely to develop a strep lung infection.
The damage to the lung cilia caused by the alcohol appeared to be intensified by the smoke, although exposure to smoke alone did not increase the likelihood that the infection would spread to the rats' lungs.
Professor Stephen Gillespie, professor of medical microbiology at University College London, said: "This provides experimental evidence for what is found in the human community.
"It is well established that smoking and excess alcohol consumption are linked with severe pneumonia.
"Streptococcus pneumoniae is the commonest bacterial cause of pneumonia so it is important to understand."
He said most cases of this infection were mild, but some could be very severe and even fatal.