Cigarette smoking is the number one cause of preventable disease and mortality in the US, leading to 480,000 deaths annually, or 20% of all deaths.

Over 16 million people live with a smoking-related illness in the US, according to figures from the Centers for Disease Control and Prevention (CDC).

In 2014, the CDC estimated that 16.8% of Americans aged 18 years and over were cigarette smokers, or around 40 million adults.

Much recent research has focused on imbalances in the gut microbiota and how they relate to immune disorders such as Crohn's disease and gastrointestinal cancers.

There are around 600 species of bacteria in the human mouth. Over 75% of oral cancers are thought to be linked to smoking, but it remains unclear whether microbial differences in the mouth affect the risk for cancer.