How does smoking make an individual more susceptible to infections?

The active smokers and those getting exposed, that is, passive smokers are at enhanced threat of bacterial infections. Getting exposed to tobacco smoke enhances vulnerability to respiratory tract infections, comprising pneumonia, tuberculosis, and Legionnaires disease.  

Smoking compromises the anti-bacterial activities of leukocytes incorporating monocytes, neutrophils, B cells, and T cells, thus, demonstrating the mechanism for enhanced risk of infections.