For infectious disease experts, the most terrifying aspect of their job is the unpredictability of emerging infectious diseases.

When Anthony Fauci first took the job as the director of the National Institute of Allergy and Infectious Diseases in 1984, there was only one that dominated everyone's minds and that was HIV/AIDS. No one knew exactly how the virus was spread, how quickly it could move from host to host — or how many it would eventually kill. Below is a copy of the slide Fauci used to show when giving talks to Congress, fellow scientists and others about this threat to humanity:

In the 34 years that have passed, the map evolved.

Fauci said in an interview that he still continues to show that same slide, but "I added one or two or three new emerging diseases every year."

The last two years have been especially busy. "Remember the measles outbreak in Disneyland, drug-resistant TB, MERS, etc. Chikungunya in the Caribbean. And now we're dealing with Zika," Fauci said. "It's now a constant battle with emerging and reemerging infectious diseases."

This is the map he showed at a hearing on Capitol Hill this week:

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