Hello, Hand foot and Mouth disease is caused by several strains of coxsackievirus viruses. But most commonly by coxsackievirus A16. If your grandson have healthy immune system and he may developed antibodies for particular strain. Thus he may not get infection from same Strain of Cockackie virus. But he may get infection by other strains of Coxsackie virus. MOst of Coxsackie virus except A16 clears from body with in a week or two weeks. Thus it can be either from A16 or Infection by another strain. There is no specific low cost investigation to differentiate the strains. But can be treated and controlled with NSAIDS . Hope this helps. Please write back for further queries. Wishing you good health.