It's just like any other opposed roll.
RQ6 p77:
An Opposed Roll is determined thus: both participants roll their respective skills. The winner is the one who gains the better level of success. If the participants score the same level of success (a standard success each, or a critical each), then the winner is the one who has the highest dice roll still within the success range of the skill.
You roll on behalf of the disease and the player rolls on their own behalf. You compare your level of success (against 45) and to theirs (against 25) and there you are.
Of course, I don't know how I'd feel, as the GM, about essentially RPing a disease, but there you are.