Fever when the mouth blisters are the following reasons: First, the most common factor is the infectious factor, the local of the mouth is particularly susceptible to bacterial, viral and especially fungal infections, the infection will cause fever, the fever will be inflammatory blisters and blood blisters in the mouth. Second, the mouth is susceptible to fungal and secondary pathogen infections when fever is present, such as anaerobic infections, which can also lead to blistering in the mouth. Thirdly, if you take antibiotics and anti-inflammatory drugs when you have a fever, you will have an allergic reaction, and blisters will appear in your mouth when you have an allergic reaction. Fourth, if there is a lack of B vitamins in the mouth when fever is present, a severe lack of vitamin B2 will also lead to clinical symptoms of blistering in the mouth.