With all the flu going round at the moment, I thought it could be useful to point out that if you become ill while on annual leave you can ask your employer to convert your annual leave to sick leave. You can then take the annual leave at a later date.
link here (scroll down to the section headed 'Sickness'.)
I've just done this for myself; after not having a single day off sick all year, I came down with flu this week.