Do not run a race if you feel unwell or have recently been unwell, even if you are raising money for charity. Most medical emergencies during races occur in people who have been unwell but do not wish to miss the event. If you feel feverish, have been vomiting, have had severe diarrhoea or any chest pains, or otherwise feel unwell, then it is unfair to you, your family, your sponsoring charity and the race support staff to risk serious illness and become a medical emergency. You are unlikely to do yourself justice, and there are many other races that you will be able to compete in.