Yesterday I had a cleaning at my dentist's office. When he was almost finishing, he said that I should use an interdental brush on two specific teeth. He asked his assistant for a sample brush to show me how to use it. As she was searching for it, he started to show me how to use it using the same tool he uses to scrap plaque from teeth. He pointed the tool towards his glove, touching his thumb very slightly with the sharp point of the tool, to simulate how I should handle the interdental brush. After that, his assistant found the sample brush, and he used it on my teeth to show me how to handle it properly.
I didn't see any blood on his glove, nor heard anything from him that could be seen as a sign of pain, so I don't think he perfurated his glove with his tool when he was simulating the process - as I said, the contact between the tool and his (gloved) thumb was soft.
But let's assume that this scenario happened: his thumb got pricked enough to drop a minuscule quantity of blood, and when he used the real brush on me, his blood got into my mouth - which was bleeding in some parts, a normal thing when we have a cleaning.
Would this be enough to infect someone with HIV in this scenario? How risky would be? Would the mix of saliva + air be enough to inactivate it? I believe it would, but I'd like to have a second opinion from the great people on this site.