From a post on Bored Panda:

“…And if you really want to talk about real life, be it COVID, the flu or common cold, you will get exposed to the virus, it will go through this process over a day or so, then you will be infectious but will not yet have symptoms. You are infecting others before you know you have millions of virus particles inside you. So if you are at work and a coworker has a cold it is good to avoid them, but if you interacted with them the day before when they had no cold, you were potentially exposed and may get the cold yourself. And as I teach students, the symptoms you experience are not due to the virus, but your immune response to the virus. Otherwise you would not be asymptomatic yet have the virus raging inside of you. When your body recognizes the foreign invader you start to get symptoms. One last tid bit, you are sick longer than you are infectious. With a cold you might be infectious till day three or four of symptoms or so, then no longer, but you still have several days of symptoms to go.”