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If you paid for cancellation 'insurance' on the cruise, there should be no problem in getting your money back, according to the terms of the insurance you chose. Typically, with a doctor's note regarding the illness necessitating your cancellation, you would receive a refund minus some fees, in most cases.
If you did not purchase the cancellation insurance, you will need to contact the Cruise Line and ask how to get a refund if you cancel due to illness, and what is required, like a note from the doctor, explaining the circumstances preventing you from traveling.
If you need contact information for your Cruise Line, please let me know it's name and I will be happy to help you find it.
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