Threading is NASTY, GROSS, AND A VERY LITIGIOUS, RISKY SERVICE WHEN THE MOUTH IS USED! I have seen many clients come in with scabby micro-'slashes' on their skin from threading/and visible at times without the MagLamp! EVERY TIME that thread is held taut and the thread is pulled (while cutting the skin), microscopic aerosolized drops of saliva and BLOOD (white and red cells!) go flying into the air... to land on surfaces, the skin, and inhaled.
I was in the dental field for almost 20 years, (as well as an OSHA compliance manager) - a field that is SO REGULATED because of saliva/blood-borne pathogenic cross-contamination. Think of a head of a straight pin: Now think of a virus particle. One million virus particles can fit on the head of a pin. Viruses are also much smaller than bacteria.