My advice is to pick up where you left off, starting with the workouts you missed. So start with the UB workouts you missed, as part of a complete week of STS (that means repeating the legs, but if you were fighting the flu, you may not have gotten as much out of it anyway), and then keep on going. If you were close to your rest week before the flu, then skip the rest week and move on to the next Mesocycle. Good luck!