Hello Although HIV can be isolated typically in low titers in a small proportion of infected individuals ;indeed there is NO CONVINCING EVIDENCE that HIV can transmit by oral route . Saliva contains endogenous antiviral factors , among these are immunoglobulins of IgA , IgG and IgM isotypes are readily detected in HIV infected individuals ( IN SALIVA ) . So even thermometer , was from the mouth of HIV infected , you can t get HIV infection by insertion of thermometer in your mouth ( as double protection was there because from your mouth and of previous patients mouth, if any). Hope this will help you.