Can you bio-engineer a virus that either produces or causes the body to produce Serotonin?

It is possible to create a bacteria or more likely an infectious agent that would produce serotonin, but such a process is beyond a virus. Viruses are to simple to produce specific protein chains unless the body produces them as a reaction to the virus. As for it being practical pain killer, that would be disastrous. First, if it were to spread amongst the population then you would have people being happy and good feeling all the time, even if there is something very wrong with their body. Pain is supposed to tell you something is wrong so that you do something about it. Say for example that you sprain an ankle- if you did not feel that, then you would not know to stay off it and let it heal, causing further damage to the ankle. Second, too much of compounds like serotonin can be extremely dangerous to human beings. Such proteins can cause muscle cell damage and malfunction and in short, could kill a person. Finally, the human body would fight off such an infection, so it would only work once.