Smokers are more susceptible to respiratory infections because

The cilia (tiny hairs lining their lungs) in their lungs are moving gently to keep mucus and dirt out of your lungs, but since the smoke gets in their lungs it damages the cilia and cause mucus build up and dirt build-up causes the higher risk for respiratory infections.

Hey there,
For a more simplified explanation, their respiratory passages lose cilia, which would otherwise remove bacteria

Hope this helps :))

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Bidk