病毒(virus)
微小、简单的传染因子，仅在动物、植物、细菌的活细胞中能够繁殖。病毒比细菌小得多，包含一个单螺旋或双螺旋的核酸（去氧核糖核酸或核糖核酸），周围是所谓「衣壳」的蛋白质硬壳，有些病毒还具有脂质和蛋白质所组成的外套。病毒形状各不相同。主要的二类是核糖核酸病毒（参阅retrovirus）和去氧核糖核酸病毒。在活细胞之外，病毒是不活跃的粒子，但在适当的宿主细胞内，病毒变得活跃起来，能够接管细胞的代谢机制，以生产新的病毒粒子。有些动物病毒产生潜伏的感染，其中病毒保持静态，在周期性短暂间隔里变得活跃，就像单纯疹病毒的情形。动物对病毒感染的反应各不相同，包括发烧、分泌干扰素、由免疫系统发动攻击等。包括流行性感冒、普通感冒和爱滋病在内的许多人类疾病，还有许多具有经济重要性的植物及动物疾病，是由病毒引起的。为了与麻疹、流行性腮腺炎、小儿麻痹、天花、风疹等病毒疾病战斗，人类已经发展出成功的疫苗。药物疗法在控制明确的病毒感染方面通常没有作用，因为抑制病毒发展的药物也抑制了宿主细胞的功能。亦请参阅adenovirus、arbovirus、bacteriophage、picornavirus、plant virus、poxvirus。
English version：
virus
Microscopic, simple infectious agent that can multiply only in living cells of animals, plants, or bacteria. Viruses are much smaller than bacteria, and consist of a single- or double-stranded nucleic acid (DNA or RNA) surrounded by a protein shell called a capsid; some viruses also have an outer envelope composed of lipids and proteins. They vary in shape. The two main classes are RNA viruses (see retrovirus) and DNA viruses. Outside of a living cell, a virus is an inactive particle, but within an appropriate host cell it becomes active, capable of taking over the cell's metabolic machinery for the production of new virus particles (virions). Some animal viruses produce latent infections, in which the virus persists in a quiet state, becoming periodically active in acute episodes, as in the case of the herpes simplex viruses. An animal can respond to a viral infection in various ways, including fever, secretion of interferon, and attack by the immune system. Many human diseases, including influenza, the common cold, and AIDS, as well as many economically important plant and animal diseases, are caused by viruses. Successful vaccines have been developed to combat such viral diseases as measles, mumps, poliomyelitis, smallpox, and rubella. Drug therapy is generally not useful in controlling established viral infections, since drugs that inhibit viral development also inhibit the functions of the host cell. See also adenovirus, arbovirus, bacteriophage, picornavirus, plant virus, poxvirus.