no, you do not lose all your marks, i was very very ill during my politics and was very sick, so i got marks on my paper, you need to tell your school, (this is an example btw) sometimes even if you have hayfever/ headache etc you can also gain marks, personally even if you are sick just like clear your head get hydrated if you can as you will feel sick on results day in the long term... try to see if you can do the exam in a seperate room if this helps also