A scanning electron micrograph of Mycobacterium tuberculosis bacteria, which cause TB. BSIP/Universal Images Group via/Getty Images hide caption

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BSIP/Universal Images Group via/Getty Images

TB or not TB?

That has been the question for nearly 150 years, when Dr. Robert Koch first identified the rod-shaped bacterium that causes tuberculosis. This serious respiratory illness is currently the world's deadliest infectious disease , killing more than a million people a year.

The most common test to determine if someone has tuberculosis hasn't really changed since the late 1800s. The process relies on phlegm.

"It's a nasty substance," says Adithya Cattamanchi , a pulmonologist at UC Irvine. "No one likes it, right? You don't like to cough it up. Health workers don't like to work with it.…