In one-stage Full mouth disinfection scaling is done in 2 visits within:

The oral cavity offers a range of different niches (mucosa, tongue, tonsils, saliva and teeth, etc.) where the bacteria can adhere and survive. The transmission of bacteria from one niche to another is likely to occur. If this transmissions occurs at high speed (nothing but translocation), it could jeopardize the outcome of periodontal therapy (because of reinfection). This can be prevented by 'One-stage Full mouth disinfection', which is nothing but scaling and root planning of all pockets in 2 visits within 24 hrs, in combination with tongue brushing with 1% chlorhexidine gel for 1 min, mouth rinsing with 0.2% chlorhexidine solution for 2 mins and subgingival irrigation of all pockets with 1% chlorhexidine gel. This concept was given by Quirynen