Hey!
I'm not sure if there is a general rule that firms follow, but I would absolutely ask. The graduate recruitment team are likely to be very understanding, especially considering you are ill, and if there are spaces later in the month then it shouldn't be too much hassle to move you to a later one - I would just send a polite email explaining the situation and ask if there is any possibility to reschedule. Even if they are unable to, at least you know that you asked!
Hope you feel better soon, and best of luck with the AC!!