For some forms of tuberculosis, the chances that an exposed person will get infected depend on whether the individual and the bacteria share a hometown, according to a new study comparing how different strains move through mixed populations in cosmopolitan cities.

Nearly a quarter of the world’s population is estimated to be infected with Mycobacterium tuberculosis (M.tb), the pathogen that causes tuberculosis, but less than 15 percent of infected individuals develop the disease.

While the vast majority of the 1.8 billion people infected with the TB bacterium never experience active disease, an estimated 5 to 15 percent do develop full-blown infections—roughly half of them within 18 months of exposure. 

People with low levels of vitamin A who live with individuals who were sick with tuberculosis were 10 times more likely to develop the disease than people with high levels of the nutrient, according to research led by investigators at Harvard Medical School.