Understanding HIV Transmission Risks: Kissing vs. Toothbrush Sharing

Question
I have been doing hiv/aids research.  There is much talk about how kissing does not pose much of a risk of transmission even  deep kissing, yet it is warned not to share toothbrushes, as that DOES pose a risk.  How can both be true.  Kissing would be more direct than a toothbrush I would think.
Would a person have to use the toothbrush RIGHT after the infected person?  Would the blood need to be visible?
I am very confused on how both those can be true. Thanks.

Answer
Good question.
Would a person have to use the toothbrush RIGHT after the infected person? yes, but even a few hours would be bad.

Would the blood need to be visible? Not really since the blood does not have to be visible for the virus to be present then brushing with the others contaminated brush introduces the virus at the traumatized gum line.