In secret labs all over the planet, researchers are playing around with the deadliest diseases that humanity has ever known. In some cases, the goal is to make those diseases even more deadly and even easier to spread. If a terrorist organization or one of our enemies really wanted to create complete and utter chaos in our society, releasing a weaponized disease in a heavily populated area would be one of the easiest ways to do that. A bomb could kill hundreds or even thousands of people, but a highly infectious pestilence could spread like wildfire all over the country and potentially kill millions of Americans. If you believe that the Iranians, the North Koreans, the Russians, the Chinese and radical Islamic terror groups all over the globe would never do such a thing because they are such decent people, you have a much higher view of humanity than I do.
Of course it wouldn’t necessarily take a large organization to pull off such an attack. These days, just a handful of people could cook something up and release it into our society if they knew what they were doing.