very low. HIV is an RNA virus which is very fragile once exposed to air. The probe that dentists use is not disposable, but it is sterilized thoroghly every time after use. The procedure is enough to kill any kind of viruses. Under one condition. you can get infected:
1. the person in front you is an HIV carrier.
2. you are treated right after him with a very short interval.
3. your dentist is very lazy and does not want to change the probe.
