Jessica is a 3 year old girl who has been brought to you by her mother who informs you that Jessica is complaining about having a sore mouth and so is refusing to eat (but drinking good amounts of water). Whilst brushing her teeth this morning she also noticed a few red marks on the roof of her mouth. Over the last 48 hours Jessica has been a bit quieter than usual and felt warm to touch but Margaret assumed she was just tired from nursery. Margaret is unaware of any other systemic symptoms or rash elsewhere. Jessica was born at term with no complications and is up to date with immunisations. She attends nursery two days a week and lives at home with her parents and three older sisters.

On examination, Jessica has a crop of vesicles on her hard palate.In addition, you can see three shallow yellow-grey ulcers on her tongue. Observations are all within normal limits.

All the following are recognised causes of her likely underlying condition, but which one is the most common?