Like viruses, respiratory bacterial pathogens have been detected in saliva during acute and symptomatic phases of respiratory illnesses, as well as in institutionalized and hospitalized, elderly individuals. However, unlike viruses, certain bacterial respiratory pathogens have been identified in the oral cavities of systemically healthy and asymptomatic individuals, especially smokers. For instance, bacteria such as Streptococcus pneumoniae can be isolated more frequently and consistently from saliva than from naso‐pharyngeal or oro‐pharyngeal swabs. These pathogens are known to reside in the subgingival crevice, the buccal mucosa and saliva.

However, exogenous pathogens are not dominant members of the oral microbiome, which is one of the most diverse in the human body with over 20 billion microbial cells. Moreover, in states of health, a robust interbacterial interaction limits or reduces colonization with exogenous pathogens. For instance, bacteriocins such as LS1 (produced by the oral commensal Lactobacillus salivarius) contribute to controlling the growth of S. aureus and S. pneumoniae, and hydrogen peroxide (which is produced by several commensal species) prevents colonization by Serratia marcescens, S. agalactiae, S. pneumoniae, Haemophilus influenzae and MRSA.

In summary, a large body of evidence supports saliva as a potential source of respiratory pathogens, however, many of these studies lack quantitative data. Therefore, there is an urgent need for studies that quantify the salivary bioload of these species in non‐infected individuals and for investigations on whether these microbial loads are high enough to create a biologically relevant infectious dose.