It's a fair question, and it hasn't been answered to my satisfaction.
What would it take to satisfy you? You don't have the basics of the knowledge you would need to understand the answer; You need highshool chemistry, biology, and mathematics to a high standard, followed by a three or four year degree in epidemiology and/or virology.
You can't get that amount of education from YouTube (or from IIDB, for that matter). You need to engage in serious study, and I suspect you need to start at the primary school science and mathematics level, which means you are looking at perhaps a decade of learning (which you should have taken advantage of when you were a child, and it was freely offered to you).
Your problem is that you want simple answers, but
reality isn't simple. If your education hadn't been so desperately lacking, you would know that.