Studies of vertebrate genomes have indicated that all species contain in their chromosomes stretches of DNA with sequence similarity to viral genomes. How such ‘endogenous’ viral elements (EVEs) ended up in host genomes is usually explained in general terms such as ‘they entered the germ line at some point during evolution’. This seems a correct statement, but is also rather imprecise. The vast number of endogenous viral sequences suggest that common routes to the ‘germ line’ may exist, as relying on chance alone may not easily explain the abundance of EVEs in modern mammalian genomes. An increasing number of virus types have been detected in human semen and a growing number of studies have reported on viral infections that cause male infertility or subfertility and on viral infections that threaten in vitro fertilisation practices. Thus, it is timely to survey the pathway(s) that viruses can use to gain access to the human germ line. Embryo transfer and semen quality studies in livestock form another source of relevant information because virus infection during reproduction is clearly unwanted, as is the case for the human situation. In this review, studies on viruses in the male and female reproductive tract and in the early embryo will be discussed to propose a plausible viral route to the mammalian germ line.

脊椎动物基因组的研究表明，所有物种的染色体中都含有与病毒基因组序列相似的DNA片段。通常用诸如“它们在进化过程中的某个时刻进入种系”之类的通用术语来解释这种“内源”病毒元件（EVE）如何在宿主基因组中终止。这似乎是正确的说法，但也很不准确。大量的内源性病毒序列表明可能存在通往“生殖系”的常见途径，因为仅靠偶然性可能无法轻易解释现代哺乳动物基因组中EVE的丰富性。在人类精液中发现了越来越多的病毒类型，并且越来越多的研究报道了引起男性不育或不育的病毒感染以及威胁体外受精实践的病毒感染。从而，及时调查病毒可用于获取人类种系的途径。牲畜中的胚胎移植和精液质量研究形成了另一种相关信息来源，因为繁殖过程中的病毒感染显然是不希望的，就像人类的情况一样。在这篇综述中，将讨论对雄性和雌性生殖道以及早期胚胎中病毒的研究，以提出一条通往哺乳动物种系的可行病毒途径。