EVERYTHING
WORTH
KNOWING
JULY/AUGUST 2019. DISCOVER 37
Viruses
VIRUSES ARE NOT ALIVE, at least in the classical
sense. While they’re made of proteins and genes like
living things, they need to interact with living host
cells to reproduce. These agents of cellular mayhem
have been the cause of history-altering outbreaks
and pandemics, from smallpox and polio to HIV and
Ebola, but were only discovered at the turn of the 20th
century. Since then, we’ve found them in nearly every
ecosystem worldwide. Viruses are, and always will be,
the world’s experts at going viral.
The good, the bad and the ugly
about these tiny tricksters.
BY ANNA GROVES
So... What Is a Virus?
Viruses are ultra-tiny packages of genetic material. A single particle,
or virion, of influenza is up to 100 times smaller than common bacteria;
you could fit some 15,000 end-to-end across the head of a pin. The
outer layer is a protective shell called a capsid; some viruses also have
a viral envelope, a second layer that helps virions attach to host cells.
The envelope can act as a sneaky cloaking device, helping virions avoid
detection by a host’s immune system.
Most viruses have just a few genes, which contain the instructions for
making new viruses in either DNA or its single-stranded relative, RNA.
But they don’t have any cellular machinery to read and execute that
genetic code. That’s where a living cell comes in. When a virus bumps
into a potential host, proteins on its outer layer interact with proteins
on the living cell’s outer membrane. If it’s the right type of cell — for
example, most influenza viruses can only infect certain cells in your nose,
throat and lungs — it’s able to latch on and inject the cell with its genetic
material. The host cell doesn’t realize the new genes are foreign, so it runs
the instructions written in the genetic code alongside its own. This tellsthe cell to make copies of the viral genome and package them up into
new viruses that burst out of the cell to find host cells of their own.
Many viruses replicate through this process. However, when an
RNA-containing retrovirus infects a host cell, the RNA is converted to
DNA and then inserted into the cell’s genome. This extra step in the
replication process creates more room for a copying error, which makes
the retrovirus more prone to mutation and rapid evolution. Researchers
have yet to develop a vaccine for the retrovirus HIV, for example, in large
part because its multiple strains keep evolving.
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Herpes
HIV
Ebola
Adenovirus
Influenza
Hepatitis B
Zika