Extremely small, metabolically inert microorganisms composed of a short strand of either DNA or RNA within a protein coat, and, in some, a surrounding envelope; they replicate only within the cells of living hosts, primarily plants, animals, and bacteria, taking over the nucleus and directing it to produce viral proteins and DNA or RNA. They frequently cause disease in their hosts. Viruses occupy a special taxonomic position: they are not plants, animals, or prokaryotic bacteria (single-cell organisms without defined nuclei), and they are generally placed in their own kingdom. In fact, viruses should not even be considered organisms, in the strictest sense, because they are not free-living, meaning they cannot reproduce and carry on metabolic processes without a host cell.極微小且新陳代謝遲緩的微生物，由一股DNA或RNA所組成，外面包覆一層蛋白質外殼，有些病毒還有一層包膜；它們僅於活體宿主的細胞中繁殖，而宿主主要為植物、動物以及細菌；它們會控制宿主體內細胞的細胞核，並使細胞核開始製造病毒蛋白質以及DNA或RNA，經常導致宿主感染疾病。病毒在分類上佔有特殊位置，因為並非植物、動物或單細胞細菌(即不具明顯細胞核的單細胞有機體)，而被歸為獨立的一界。實際上，病毒甚至不應被視為有機體，因為它們不能單獨存在，如果沒有宿主，它們便無法繁殖與進行新陳代謝。