Erythema nodosum is an inflammatory disease occurring in the subcutaneous fat, the cause of which may be related to infection, drugs, autoimmune diseases, etc. It can be associated with a variety of systemic diseases and is not in itself contagious. However, if erythema nodosum is caused by infectious diseases, the infectiousness of the disease should be noted. The clinical manifestation of erythema nodosum usually consists of symmetrically distributed red nodules on the front side of the lower leg, with localized elevated skin temperature and self-conscious pain and pressure. It may also be accompanied by general discomfort, such as headache, fatigue, fever and other symptoms. Erythema nodosum caused by infectious factors is mainly caused by streptococcus, intestinal bacteria, Mycobacterium leprae, HIV and other infections causing type IV metaplasia, and is not a disease directly induced by bacteria and viruses, so erythema nodosum itself is not infectious; diseases caused by Mycobacterium leprae, gonococcus, HIV, etc. can also be accompanied by erythema nodosum, and because these primary diseases are infectious, so we should pay attention to the infectiousness of this primary disease. However, the skin manifestation of erythema nodosum is not contagious. The main treatment of erythema nodosum is to actively search for the cause of the disease and take appropriate treatment, such as the combination of infection, the application of the corresponding anti-infective drugs. In addition, painkillers can be applied for symptomatic treatment, and if necessary, glucocorticoids, combined with immunosuppressants, such as hydroxychloroquine and tretinoin, can also be applied for treatment, and attention should be paid to rest and reduce activities during the acute period.