Living things are constantly evolving. It’s how we adapt to our changing environment and pass down the most desirable traits for survival. This is a slow process happening over the course of many generations and thousands of years. Bacteria and viruses undergo this process as well. Additionally, these microscopic organisms reproduce at a rate much quicker than humans. Because of this, they can evolve much more rapidly. There are two main mutations or changes that can allow a disease to become a pandemic.