The "Spanish flu" virus of 1918 killed an estimated 50 million people, more than all the deaths in World War I combined. While global public health efforts have greatly improved mortality rates in more modern outbreaks, experts say the next pandemic is a matter of "when," not "if." In this episode, Ian Bremmer takes a look how diseases spread and become global. His guest, Dr. Anthony Fauci, is a leading epidemiologist and director of the National Institute of Allergy and Infectious Diseases at the NIH.

Dr. Fauci breaks down some of the biggest health threats facing the world today: HIV/AIDS, Ebola, malaria, tuberculosis, influenza, and the recent rise in cases of measles brought on by the misguided anti-vaccine movement.

Also on the show: Five years after his Ebola diagnosis made international news, NYC's Dr. Craig Spencer tells GZERO Media what he learned from the experience and what his life is like today.

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GZERO World with Ian Bremmer is returning to your screens this week, kicking off Season 9 in a summer of sweltering global tensions. The United States is celebrating its 250th birthday, a war has reshaped the Middle East, AI is forcing humanity to confront profound ethical choices, and democracies around the world are bracing for what comes next. Host Ian Bremmer is here to make sense of it all.