Human immunodeficiency virus is transmitted through blood, sexually, HIV can also be transmitted from mother to child at birth; You can get infected if you brush your teeth inaccurately, touching the gums to micro-scratches.
It is possible, but only in theory, but in practice it is very difficult to do. There are many factors that must coincide. HIV positive and you should have bleeding sores in your mouth and you should use your toothbrush immediately afterwards and without washing it under water. In an open environment, the virus dies in minutes. As far as I know, not a single case of transmission by this route has been recorded in several decades of HIV history.