Medical scientists at Trinity College Dublin and St James's Hospital in Ireland have unlocked the mechanism underlying the connection between smoking and Tuberculosis (TB).

TB is an infectious disease that kills 1.5 million people each year, and smoking is the biggest driver of the global TB epidemic.  This discovery will considerably strengthen anti-smoking efforts to control TB and uncovers new therapy and vaccine options for TB.

Tuberculosis spreads from person to person by inhaling infected droplets made when the TB sufferer coughs. The World Health Organization has designated TB a global emergency. Nearly 9 million people fall ill with TB each year, and it is the greatest killer worldwide due to a single bacterial infection. Many countries have recurring outbreaks and multi-drug resistant TB cases.

"This study provides evidence which explains the link between smoking and TB and should considerably strengthen anti-smoking efforts to control TB," Joseph Keane, senior author of the study, said in a statement. "However, the widespread emergence of multi-drug resistance TB means we badly need new therapy and vaccine options for TB. We are already applying the findings of this study to develop new treatment options."

For the study, the research team collected and analyzed data from smokers, ex-smokers and non-smokers attending the bronchoscopy suite at St James's Hospital in Dublin.

They found that the white blood cells located in the lungs of smokers and ex-smokers, which are responsible for fighting infections, showed a weakened response to the TB infection. In the smoker's lungs, these cells malfunction, and fail to make the chemical messengers that would normally fight the TB bacteria. In fact, the researchers found that these cells suppress the lungs' immunity after infection, which gives the TB bacteria a chance to take over.

"TB remains a huge global health problem, affecting millions worldwide. It has been known for some time that smokers are more susceptible to getting TB and nearly 80 percent of the world's one billion smokers live in countries of high TB prevalence," Keane said. "Therefore, while HIV is a key driver of the disease, numerically, smoking is more prevalent than HIV, making smoking the biggest global driver of the TB epidemic."

The findings are detailed in the American Journal of Respiratory and Critical Care Medicine.