It seems implausible to me that we could engineer a virus that was able to replicate in humans prolifically enough to be spread person to person but not cause a

It seems implausible to me that we could engineer a virus that was able to replicate in humans prolifically enough to be spread person to person but not cause any damage. My understanding is that viruses replicate by hijacking human cells and I don't think those cells survive the ride. Feeling sick is usually your immune system reacting and killing off the virus. If it didn't react at all, the virus would eventually kill enough cells to kill the host. The only way to not get sick from a viral infection is for your immune system to react quickly before the virus spreads too much, which would kill the virus before the host had enough of it to start shedding it and become infective to others.