Why HIV cannot spread through saliva?
Saliva contains fibronectin which inhibits HIV as a result it can't spread
Fibronectin is a high-molecular weight (~440kDa) glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins.Similar to integrins, fibronectin binds extracellular matrix components such as collagen, fibrin, and heparan sulfate proteoglycans (e.g. syndecans).
It has a role in inhibiting colonization by pathogenic bacteria and virus.
Probably it is another reason why Oral Polio Vaccine was found to be ineffective initially in tropical countries as it caused inhibition of the polio virus as well.