“…In the oral cavity, there may be from 10 8 to 10 10 CFU per gram of saliva ( Lazarevic et al, 2009 ). It should be noted that the oral microbiota even in healthy people varies drastically across location in the oral cavity, time of day, hydration, what and when the person ate, oral hygiene, age, smoking status, and so on ( Aas et al, 2005 ; Bik et al, 2010 ; Cameron et al, 2015 ; Leake et al, 2016 ; Hall et al, 2017 ; Belstrøm, 2020 ; D’Angiolella et al, 2020 ). In radiation exposures, oral side effects such as xerostomia (dry mouth) are seen in patients receiving external beam radiotherapy to the head and neck ( Wijers et al, 2002 ; Dirix et al, 2006 ) and radioiodine therapy ( Alexander et al, 1998 ; Solans et al, 2001 ; Jeong et al, 2013 ; Hollingsworth et al, 2016 ).…”