Lung nodule 3mm with low fever is serious and related to lung infection, autoimmune disease, lung tumor and other factors. 1. Lung infections: lung infectious diseases can easily cause pulmonary nodules, such as Mycobacterium tuberculosis infection, fungal infection, bacterial infection, which can easily cause low fever and other symptoms, which are more serious. 2. Autoimmune diseases: rheumatoid arthritis and systemic lupus erythematosus are common, and the disease will easily accumulate in many organs in the late stage, showing lung nodules, accompanied by low fever and other symptoms, which is a more serious disease. 3. Lung tumor: lung nodule 3mm may be benign tumor or malignant tumor. The disease is more serious when there are symptoms of low fever for a long time. If there is a lung nodule 3mm with low fever, it is recommended to consult a doctor in time, clarify the cause of the disease with the help of the doctor, and carry out targeted treatment or therapy.