Is that completely certain?

In the case of COVID-19, you can infect people before developing COVID-19 symptoms. Anecdotally, I believe that I contracted COVID-19 and infected some friends before I was even aware of the infection building up inside my body. You would only get symptoms when the body starts responding to the infection.

Now the question is: Can a virus exist that allows infection by close contact (e.g. talking to someone) but doesn't have severe symptoms? That is certainly the way I experienced COVID-19, but it is not a universal truth for the disease.