When the genus Pandoravirus was first identified in 2013, it was crowned as the world’s largest virus, at 0.5 micrometers in length and 1 micrometer in diameter. The two known pandoravirus species, Pandoravirus salinus (left image) and Pandoravirus dulcis, were discovered in sediment samples taken thousands of miles apart—off the coast of Chile and in Melbourne, Australia, respectively. P. salinus, the larger of the two, has a genome that’s nearly 2.5 million nucleobases long. Roughly 94 percent of its genes bear no discernible resemblance to any other known organism, which could uncover metabolic pathways that might revolutionize the biotech and pharmaceutical industry. The doctors who described the new viruses said, as far as they know, the pandoraviruses are not pathogenic to humans (although they can parasitize amoebas).

Norovirus (right image) is the world’s leading cause of viral gastroenteritis. Although it’s certainly harmful to humans, recent research on its rodent equivalent –which typically does not cause its host to become ill—shows that it could have its uses. When norovirus-infected mice were given antibiotics that wiped out their gut flora, they exhibited more resistance to pathogens and chemical irritants than their control counterparts. This suggests that viruses might be able to defend against infection in the absence of beneficial bacteria.