Both petechiae and petechiae are caused by the rupture of capillaries and bleeding, which manifests itself as bright red or greenish-purple spots on the skin that do not fade when pressed. Clinically, they are called petechiae if they are less than 2mm in diameter and petechiae if they are larger than 2mm in diameter. If the petechiae are so densely distributed that they appear to be fused into patches, they can also be called petechiae. There are three reasons why capillary rupture and bleeding can occur: the first one is due to the problem of blood vessel itself; the second one is due to the problem of coagulation function; the third one is due to the problem of platelet function and quantity, the most common clinical situation of petechiae and petechiae is allergic purpura.