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Other coronaviruses were responsible for deadly
outbreaks of Serious Acute Respiratory Syndrome
(SARS) in china in 2003 and Middle East Respiratory
Syndrome (MERS) from 2012. These viruses mutate
relatively often in ways that allow them to be
transmitted to humans.
An Infinity of Viruses
If you get sick with the flu, for example, every infected
cell in your airway produces about 10,000 new viruses.
The total number of flu viruses in your body can rise to
100 trillion within a few days. That's over 10,000 times
more viruses than people on Earth.
If there can be so many viruses in a single person, how
many viruses are there in total on our planet? As in over
10 million times more viruses than there are stars in the
universe. As in, if you were to stack one virus on top of
another, you'd create a tower that would stretch
beyond the moon, beyond the sun, beyond Alpha
Centauri, out past the edge of the Milky Way, past
neighbouring galaxies, to reach a height of 200 million
light-years.
Now it seems that I may have been lowballing that
number.
Scientists don't come up with a number like 1031 by
counting every single virus on Earth. They take surveys in
different environments-the soil, the water, the ocean
floor-and then extrapolate. As they take more samples,
they can adjust their estimate up or down.
It used to be that scientists could only conduct these
surveys by squinting down a microscope and actually
spotting the individual viruses. That turns out to be a
bad way to count viruses, in part because their hosts-
such as bacteria and protozoans-often don't grow in the
artificial confines of a laboratory. Without viable hosts,
the viruses can't reproduce, and so they go unnoticed.
Things got a lot better once scientists found a new way
to count. Instead of looking for full-fledged viruses, they
just looked for pieces of their DNA. The ocean, once
considered a desert for viruses, turned out to be a broth
of viral DNA. Given the sheer volume of the ocean, most
of the world's viruses reside there. And most of the
viruses that scientists discovered in the ocean turned
out to be parasites of bacteria, known as
bacteriophages.
But scientists have long known that there's another
kind of virus out there, one that uses a somewhat
different molecule for its genes.
DNA is a double helix, which encodes genes along both
strands. When our cells make a protein from a gene,
they make a copy on a single-stranded molecule called
RNA. DNA-encoded genes can also make RNA
molecules that do other jobs, such as sensing the
concentration of various elements inside a cell.
In some viruses, RNA also takes on DNA's job and
encodes genes. Influenza is just one example of the
many RNA viruses out there (or in you). Unlike
bacteriophages, RNA viruses almost never infect
bacteria. Instead, they infect us, plants, fungi,
protozoans-all members of the same great lineage of
life known as eukaryotes.