A question I often get in class is: “Are there viruses that infect other viruses?”
The answer is still “no” but a recent discovery reported in Nature does starts to blur that line. A newly identified virus, called Sputnik, tags along with another virus called Mamavirus (so called because its bigger than the previous record holder for large viruses, Mimivirus).
Interestingly, Sputnik virions can be found within Mamavirus virions, so they travel together. Since Sputnik can not replicate with in the Mamavirus virion, this is not an active infection of Mamavirus, but rather it is a passive particle traveling within the Mamavirus virion.
Importantly, Sputnik is a parasite of Mimivirus. It can only replicate in ameoba that are co-infected by Mamavirus. Parasites live off of other organisms, with a deleterious effect on the host, such as reduced nutrient uptake or growth rate. Viruses are parasites – infected cells usually halt all or most cellular activity and eventually die, the most deleterious effect of all. Sputnik is a parasite of Mamavirus it replicates at the expense of Mamavirus. Cells infected with both viruses produce less 70% less infectious Mamavirus particles than a cell not co-infected with Sputnik. A few other viruses have been identified that require co-infection with another virus (Hepatitis D virus for example) but these do not reduce the infectivity of the host virus so are not really parasites of the other virus.
Mamavirus hijacks a cell to replicate. Sputnik hijacks Mamavirus for transportation and its cell-hijacking capabilities. It is the first described virus of a new group of viruses called virophages.