They are some of the most lethal substances on earth - transmitted through food, water, the people around you or even the air you breathe. These are bioweapons, strains of bacteria and viruses that include the most feared diseases in human history. Now, imagine that you have just become the leader of your nation and new intelligence confirms that terrorists are trying to make and unleash a biological weapon in one of your cities. What are their chances for success? National Geographic reveals a surprising picture of what could happen during a biological attack by examining potential scenarios, vulnerabilities, historical instances and the steps we can take to protect ourselves from the worlds most dangerous pathogens.