巨型DNA在光学显微镜下可见，它们的 编码的蛋白质比一些 或细胞内寄生的真核生物还多。目前已发现两种截然不同的 ，他们都能感染单细胞原生生物，如阿米巴原虫（Acanthamoeba）。一方面，巨型病毒（Megaviridae）具有大的伪二十面体（pseudoicosahedral）衣壳包围着大小以兆为单位、富含腺嘌呤和胸腺嘧啶的基因组；另一方面，最近发现的潘多拉病毒（Pandoraviruses）具有微米级的双耳罐状颗粒和高达2.8 MB富含鸟嘌呤和胞嘧啶的基因组。
最近研究人员从西伯利亚永久冻土中分离到了第三种巨型病毒——西伯利亚（Pithovirus sibericum），其形态更类似于二十面体结构，直径约1.5μm，基因组大小为600 kb，富含AT。西伯利亚阔口罐病毒是从3万年前的冻土中分离到的，表明无论是从全球变暖或工业开采极地地区的永久冻土层的融化，都有可能对人类或动物的健康产生威胁。
Legendre M, Bartoli J, Shmakova L, et al. Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology. Proc Natl Acad Sci USA. 2014. doi:10.1073/pnas.1320670111
The largest known DNA viruses infect Acanthamoeba and belong to two markedly different families. The Megaviridae exhibit pseudo-icosahedral virions up to 0.7 μm in diameter and adenine–thymine (AT)-rich genomes of up to 1.25 Mb encoding a thousand proteins. Like their Mimivirus prototype discovered 10 y ago, they entirely replicate within cytoplasmic virion factories. In contrast, the recently discovered Pandoraviruses exhibit larger amphora-shaped virions 1 μm in length and guanine–cytosine-rich genomes up to 2.8 Mb long encoding up to 2,500 proteins. Their replication involves the host nucleus. Whereas the Megaviridae share some general features with the previously described icosahedral large DNA viruses, the Pandoraviruses appear unrelated to them. Here we report the discovery of a third type of giant virus combining an even larger pandoravirus-like particle 1.5 μm in length with a surprisingly smaller 600 kb AT-rich genome, a gene content more similar to Iridoviruses and Marseillevirus, and a fully cytoplasmic replication reminiscent of the Megaviridae. This suggests that pandoravirus-like particles may be associated with a variety of virus families more diverse than previously envisioned. This giant virus, namedPithovirus sibericum, was isolated from a >30,000-y-old radiocarbon-dated sample when we initiated a survey of the virome of Siberian permafrost. The revival of such an ancestral amoeba-infecting virus used as a safe indicator of the possible presence of pathogenic DNA viruses, suggests that the thawing of permafrost either from global warming or industrial exploitation of circumpolar regions might not be exempt from future threats to human or animal health.