医学百科 The Medicine Book (Big Ideas Simply Explained)
When DNA is copied, it is copied near perfectly, but when RNA viruses such as coronaviruses and flu replicate themselves in order to infect other cells, there are often misprints.
So like when I, I played ultimate Frisbee in college and we would buy, you could buy like a bulk bag of just like random misprint Frisbees for way cheaper than you would buy them.