You are seeing a 10-year-old boy and his mother in a GP setting. He has presented with a 24 hour history of a sore throat, a low-grade fever and, loss of appetite and a sore mouth. On examination you note that there are erythematous macules in the mouth and there are also erythematous macules and papules with a central greyish vesicle on the dorsum of the hands and heels. Given the diagnosis, what is the most common causative organism associated with this illness?