Researchers begin a slow and cumbersome descent, deep into the Grootboom Cave, hindered by head-to-toe protective suits, but those suits are the only things standing between safety and exposure. Thousands of bats live inside this cave, and not far from the cave's entrance is the city of Johannesburg, South Africa, a heavily populated area. Any number of these bats could potentially be carrying a deadly pathogen. Known as zoonotic, these are diseases that can jump from animals to humans, responsible for viruses like SARS and MERS. And in a time where international travel has connected the world more than ever before, it takes a coordinated, global effort to prevent a widespread outbreak—starting with the teams of researchers in caves like this one. CNN's Dr. Sanjay Gupta and David McKenzie take us from the caves of South Africa to the Centers for Disease Control's headquarters monitoring this work all over the world, following virus hunters and disease detectives as they work to protect us all.