Hi, I always had this doubt because it's not clear (for me at least) from the books.
After being injured by Khal Ogo, Mirri Maz Duur puts a poultice on Drogo's chest cut, which gives him trouble from the beginning until causing a several infection that would end up with his life. After that, we know that the maegi tricks Daenerys into a black magic ritual that she knew would not bring Drogo to his original state, but rather a living corpse, as a vengeance on the Khal that killed her people. One can interpret that she planned to damage Drogo since the very first moment by poisoning the poultice, which would explain why a regular cut infected so bad, but after the dark ritual she just confesses her bad intentions about bringing Drogo back to life, but not that she actually posined her. Am I right?
I have searched online but couldn't find an actual answer nor a GRRM interview in which he confirms it, so I thought that you may know the answer.