A new group of picornaviruses is disclosed. The picornaviruses of the invention comprise in the non-coding region of their viral genome a nucleotide sequence which corresponds to cDNA sequence (I) or homologous sequences having at least 75% homology to the SEQ ID NO:1, and they cause mammalian disease. Further aspects of the invention comprise a protein corresponding to a protein of the picornaviruses, antiserum or antibody directed against a protein of the picornaviruses, antigen comprising a protein of the picornaviruses, diagnostic kits, vaccines, use of the picornaviruses in medicaments, particularly for the treatment or prevention of Myocarditis, Cardiomyopathia, Guillain Barre Syndrome, and Diabetes Mellitus, Multiple Sclerosis, Chronic Fatique Syndrome, Myasthenia Gravis, Amyothrophic Lateral Sclerosis, Dermatomyositis, Polymyositis, Spontaneous Abortion, and Sudden Infant Death Syndrome, and methods of treatment of diseases caused by the picornaviruses. TABLE-US-00001 SEQ ID NO: 1 (Ljungan 87 012) (I) AGTCTAGTCT TATCTTGTAT GTGTCCTGCA CTGAACTTGT 50 TTCTGTCTCT GGAGTGCTCT ACACTTCAGT AGGGGCTGTA CCCGGGCGGT 100 CCCACTCTTC ACAGGAATCT GCACAGGTGG CTTTCACCTC TGGACAGTGC 150 ATTCCACACC CGCTCCACGG TAGAAGATGA TGTGTGTCTT TGCTTGTGAA 200 AAGCTTGTGA AAATCGTGTG TAGGCGTAGC GGCTACTTGA GTGCCAGCGG 250 ATTACCCCTA GTGGTAACAC TAGC