suspected, or if they just colonize people who are already sick. Scientists have also found
mimiviruses and related giant viruses far from hospitals. They are actually common in the world’s
oceans, where they infect algae and perhaps even corals and sponges. Until now, scientists have
realized, these giant viruses have been hiding in plain sight.
Newly discovered viruses like the mimivirus are forcing scientists to rethink what it means to be a
virus in the first place. Their old rules, once so ironclad, are buckling. And as scientists debate what
it means to be a virus, they are debating an even bigger question: what it means to be alive.
Scientists have long seen a huge gulf dividing viruses from “true” living things—bacteria,
protozoans, plants, animals, and fungi. Many pointed to the tiny number of genes in viruses, arguing
that there was no way for them to gain more because of their peculiar way of reproducing. Because
viruses hijack cells to make new viruses, they are sloppy about copying their genes. They don’t carry
their own repair enzymes that can fix errors, for example. As a result, they are much more vulnerable
to lethal mutations. If a virus accumulated thousands of genes, its high mutation rate would wipe it
out.
The sizes of virus genomes offered some good reason to believe this was actually true. Viruses
carry genes encoded either in DNA, or its single-stranded version, RNA. For a number of reasons,
RNA is an inherently more error-prone molecule to copy. And it turns out that RNA viruses, like
influenza and HIV, have smaller genomes than DNA viruses.
Forced to carry tiny genomes, viruses could not make room for genes that did anything beyond
make new viruses and help those viruses escape destruction. They could carry genes to let them eat,
for example. They could not turn raw ingredients into new genes and proteins on their own. They
could not grow. They could not expel waste. They could not defend against hot and cold. They could
not reproduce by splitting in two. All those nots added up to one great, devastating Not. Viruses were
not alive.
To be alive, many scientists argued, required having a true cell. “An organism is constituted of
cells,” the microbiologist Andre Lwoff declared in a lecture he gave when he accepted the Nobel
Prize in 1967. Lacking cells, viruses were considered as little more than cast-off genetic material that
happened to have the right chemistry to get replicated inside cells that were truly alive. Scientists
could purify viruses down to crystals, the same way they could crystallize salt or pure DNA. No one
could ever crystallize a maple tree. In 2000, the International Committee on Taxonomy of Viruses
declared that “viruses are not living organisms.”
In the decade following that declaration, a number of scientists rejected it outright. The old rules no
longer work well in the face of new viruses. Mimiviruses, for example, went overlooked for so long
in part because they were a hundred times bigger than viruses are supposed to be. They are also
loaded with far too many genes to fit old-fashioned notions of a virus. Scientists don’t know what
mimiviruses do with all of their genes, but some suspect that they do some rather lifelike things with
them. Some of their proteins, for instance, look a lot like the proteins our own cells use to assemble
new genes and proteins. When mimiviruses invade amoebae, they don’t dissolve into a cloud of
molecules. Instead, they set up a massive, intricate structure called a viral factory. The virus factory
takes in raw ingredients through one portal, and then spits out new DNA and proteins through two