Freezing viruses in time.

The 2014 outbreak of Ebola in West Africa has generated understandable interest in how long viruses can survive outside of a host. Is it a matter of minutes, several hours, a few days? How about 700 years? Remarkably, a new study in PNAS by Ng et al. (1) shows that viruses can survive intact in ice patches for at least this period and retain their infectivity. Ng et al. (1) based their study on a 700 ± 40-y-old caribou fecal sample cored from a permanent ice patch in the Northwest Territories of Canada. From this sample, the authors recovered two novel viruses: a singlestranded DNA virus that they termed Ancient caribou feces-associated virus (aCFV), and the partial sequence of a single-stranded RNA virus called the Ancient Northwest Territories cripavirus (aNCV). That aCFV is a distant relative of the geminiviruses, a wellknown group of plant viruses, strongly suggests that the virus was present in the plant diet of the caribou. aNCV is more intriguing, in part because it is most closely related to a group of insect RNA viruses (dicistroviruses) not known to infect plants, such that an insect may have been consumed with the vegetable matter or died on the fecal sample. Studies of “ancient” RNA viruses are also few and far between, and many have question marks over their authenticity. Indeed, the pioneering work recovering the genome of the influenza virus associated with the notorious pandemic of 1918–1919 still serves as the benchmark for work in this area (2, 3). Although aCFV and aNCV are not the oldest viral genomes obtained, and earlier this year it was proposed that a replication-competent DNA virus (Pithovirus sibericum) could be recovered from Siberian permafrost dating to 30,000 y ago (4), they are undoubtedly some of the most believable.