ckHenson, there is a colonization event where you find Festron and if you do not pick the choice which spaces them you will have them in your empire. Because of their ability I do not play with them in my games.
Note : Trade routes WILL bring them in your empire. You are Empire A trading with the Infected Empire C who is NOT Festron but does trade with them. Thus even though you did not meet them you brought them home to hive. Really you should just nuke them from orbit...its the only way to be sure!
Check if the infected planet has any trade route this could be a source.
After all is said, blame the devious Cari as this atrocity it is all hers! (Good job Cari)