SAN FRANCISCOA virulent strain of soy flu has been traced to a single tofurkey at a Bay Area food-processing factory. 'An investigation of Green Earth Foods has located the bird-shaped loaf of firm bean curd from which the infection originated,' said Dr. Julie L. Gerberding, director of the Centers for Disease Control and Prevention. 'To prevent further spreading of the disease, all tofurkeys in Northern California are being quarantined and destroyed.' Gerberding said it appears that the soy virus was not transmitted to the factory's Spaghetti & Wheatballs Microwaveable Entree division. 