Theoretical, if blood transfered to open sores in your mouth. No reported cases of transfers exclusively via that route.
Hepatitis and baterial transfers are the greater concern there.
This came up recently on another thread.
Our resident medically literate go-to BM, Rxpharm, responded to a post regarding the risks of rimming.
As Veveron indicated, Bro. RX confirmed that hepatitis is indeed a backside landmine waiting to explode on the first unwary tongue unlucky enough to tickle the trigger there. (And as jfrank indicated, the potential for bacterial infections ought to be obvious. That's why mom taught us to wash with soap and water after poo'ing.
RX indicate that bacterial and viral (hepatitis) risks can be greatly reduced by the simple act of washing the recipients bung-hole thoroughly with lots of soap and warm water. For that reason, and as a matter of routine, i insist n a soapy shower with my intended....and really get focused on the target areas, even inserting a soapy finger into the ringpiece several times.
With the right spirit of playfulness, this can be considered part of the fun.
The risk of course is thus "mitigated", never eradicated. For eradication of risk, abstinence or monogamy are the only foolproff methods of which i am aware.