Blisters on the mouth may be caused by virus infection, such as herpes labialis, which often occurs at or near the lips. Generally, there are many clustered blisters, and a single blister is rare. The disease caused by this virus infection is often easy to appear when people's resistance drops, such as after catching a cold and having a fever, when feeling anxious or when using immune preparations. In addition, if you get angry, it may also lead to blisters on your mouth, such as eating spicy and greasy food on your diet, or endocrine disorders caused by staying up late.