You need medical clearance...
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If you have an illness that you believe to be contagious and transmissible.
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If you are unable to take care of yourself and require special assistance.
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If you have a medical condition that may be affected by flying.
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If you believe that you may cause discomfort to other passengers, due to your physical or mental condition.
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If you believe that there may be a potential risk to the safety or punctuality of the flight or a risk of causing the flight to have to be diverted or having to carry out an emergency landing.
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If you are 36 weeks pregnant or more and need to travel (or 32 weeks, if it's a multiple pregnancy with no complications).