Erythema behind the ear is one of the clinical manifestations of infectious erythema. It is a mild febrile infectious disease caused by a small viral infection, mostly seen in children under 12 years of age. It is prevalent in spring. Viral particles can be seen by electron microscopy during the acute phase, and specific IgM antibodies are detected in the serum during the acute phase or early recovery. Specific viral DNA can be detected in serum and throat secretions. 1. Blood test: peripheral blood leukocytes may be mildly reduced or normal, and hemoglobin and platelets may be reduced in those with remitting crisis. 2.B19DNA test: PCR method can detect DNA from patient’s serum, CSF and lesion tissue, which is a means to confirm the diagnosis. 3.Immunological examination: B19 antigen in serum can be detected by radioimmunoassay or ELISA, and IgM antibody of B19 is more frequently used, which can reach the peak in 1 week of disease and decline after 2 months, so it has the diagnostic value of presenting infection; IgG antibody appears a little later and can last for a long time, which has a protective effect. Pathological changes: epidermal cell edema, dermal papillary layer vasodilatation, endothelial cell swelling, histiocyte infiltration around blood vessels, hair follicles and sweat glands, but chronic inflammatory changes, no diagnostic value. 1.It needs to be differentiated from rubella and measles. 2, need to be distinguished from skin, vascular damage diseases. (1) Vascular purpura (vascularpurpura): patients have pediatric and adult. First appear fever, sore throat, runny nose, etc., and then rash appears after 48h, which is characterized as vascular purpura. It appears first on the extremities and then extends to the trunk, neck and even the face. The purpura lasts for several days and may be accompanied by short-term leukocytopenia and thrombocytopenia. Some patients have abdominal pain or large joint pain. The histological examination has both necrotizing vasculitis and non-necrotizing vasculitis manifestations. (2) Infectious erythema (erythemainfectiosum): This disease is also known as the fifth disease. 1889 Tschamer has done a detailed description of this disease. There were several epidemics around the world, and it was not until 1981 that HPV-B19 was identified as its etiologic agent. The clinical manifestation of infectious erythema in children starts with fever, general malaise, sore throat, runny nose, etc. After 2 to 3 days, a rash appears, mostly on the face, which soon fuses into patches and is accompanied by mild edema, forming a “slap face” with a special appearance. The rash soon extends to the trunk and extremities. The rash starts as a maculopapular rash and then fades to a reticular or lace-like pattern in the middle. The rash may be aggravated by sun exposure, exercise, and bathing, and is accompanied by pruritus. The rash lasts 2 to 4 days and the pigmentation may fade after a few days, with a total duration of 5 to 9 days.