Pretty safe bet that we aren't. Three games to my knowledge have done a virus that spreads by being near other players. Horizons, World of Warcraft, and Everquest 2. The outcry in all three games was tremendous and it probably won't be done again in any of them.
It is a really neat gameplay mechanic and a great source of RP, but it is also very open to griefing and that has a major tendency to drive players away. Which is the last thing we want to do.