A 4-year-old child presents with headache, nausea, pain when swallowing. Body temperature is 39°C. Examination of the oral cavity revealed extremely hyperaemic mucous membrane and tonsils. On the first day of disease the child's tongue was furred with greyish coating, on the second day the tongue was found to be self-purified. On the third day the tongue was smooth, of crimson colour, with hyperaemic fungiform papillae. What is presumptive diagnosis?