A method of prophylactic and/or therapeutic treatment of a mammal for a disease that is caused by a Ljungan virus infection, such as Myocarditis, Cardiomyopathia, Guillain Barre Syndrome, and Diabetes Mellitus, Multiple Sclerosis, Chronic Fatigue Syndrome, Myasthenia Gravis, Amyothrophic Lateral Sclerosis, Dermatomyositis, Polymyositis, Spontaneous Abortion, Intrauterine Death, Preeclampsia, Sudden Infant Death Syndrome, Bell's (facial) paralysis, Addison's disease, and Pernicious anemia, is described. An antiviral compound effective against a Ljungan virus, such as a compound effective against a picornavirus, e.g. Pleconaril or a derivative thereof, is used for the preparation of a medicament for the treatment of a disease in a mammal that is caused by a Ljungan virus infection, to eliminate or inhibit proliferation of the virus in the mammal and at the same time prevent and/or treat the disease in the mammal. A composition for treatment of a mammal for a disease caused by Ljungan virus infection including an antiviral compound, antiserum, and an interferon. A method of treatment of a mammal for a disease caused by a Ljungan virus infection.