Why Herpes Is the Most Talented Virus Ever

[♪ INTRO] Getting sick is never pleasant, but at least
many viral infections are one-and-done. We’ve talked before about how the measles
virus is the poster child for lifelong immunity. Once your immune system recognizes it, it
never forgets, and it will not hesitate to kick it to the curb
if it shows its face again. Unfortunately, however, there’s also a poster
child for lifelong infection: herpesviruses. These viruses are so good at sneaking past
our immune system and hiding out in our cells that once we have
‘em, we… have ‘em. Because herpesviruses have some unique features
that make them master squatters. Which seems kind of embarrassing for our immune
systems — but actually, we may have evolved to coexist. There are eight members of the herpesvirus
family that routinely infect humans. These include herpes simplex virus 1 and 2,
which cause oral and genital herpes, as well as the viruses that cause chickenpox
and shingles, and most — but not all — cases of mononucleosis, or mono. Herpesviruses are nearly everywhere. It’s thought that more people on this planet are infected with herpes simplex 1 than not. Herpesviruses are known for being unusually
large and complex, as viruses go.

Their genomes are so large, with so many different
genes, that they can make dozens or even hundreds
of proteins. That’s way more than other viruses — influenza
virus genomes, for instance, only make twelve proteins. These proteins give herpes all kinds of tools
that let it infect a host…forever. Like a gang of thieves in a heist flick, these
viruses slip past all of the traps and defenses our body puts out for them, one
by one.

First, herpesviruses have to get past our
innate immune system — the part of our immune system that holds off
new, unknown invaders — without being noticed. It’s a sneaky move known as immune evasion. The innate immune system’s first line of
defense against new threats starts with the production of proteins that
kick cells into bouncer mode, setting off a huge signal cascade that ultimately
prevents viruses from using host cells to replicate.

But herpesviruses reduce the cell’s production
of those bouncer proteins, which dampens cells’ first line of defense. But cells have backup defenses against viruses
that can get past this pathway. Which herpes also has an answer to. Herpesviruses can integrate themselves into
the host’s DNA, which can cause damage to host’s genome. And cells are normally super vigilant about
monitoring for DNA damage. But herpes can convince them to overlook it. For example, herpes simplex virus 1 produces
proteins that basically sabotage the cell’s DNA damage warning and repair
machinery and take it over. The next trap seems inescapable: When all
else fails, a cell making a last-ditch effort to contain
a virus will basically kill itself. But you probably know where we’re going
with this by now. Herpes has proteins that can evade the cell’s
self-destruct signals.

Once it gets past the innate immune system, herpes has to stay out of trouble long enough
to set up camp within the host’s cells. While it’s working on that, herpes is in
what virologists call the lytic phase — it’s actively reproducing and sending out
new viruses. And that gives the body’s adaptive immune
system — the one that develops antibodies that can
bind to viral molecules, or antigens — a chance to recognize those viral particles.

But herpes puts the brakes on this process,
too. Normally, there are specialized cells that
grab bits of viral antigens and show them to the immune system, like molecular
snitches. But herpes gums up the cellular machinery
that would normally move antigens into position for the immune system to recognize
it. Still, the host does eventually develop antibodies. Only by this time, it is too late. The antibodies can’t clear the infection
because herpesvirus is already hunkered down inside
the host’s cells. This is called the latent phase. The virus packs up its genome in a tight,
circular form, and only certain parts of it continue to be
expressed. For this to work, the cell it’s using as
a secret hideaway has to stay alive. So the viruses typically target types of cells
that last a long time and perform critical functions, like neurons
and immune cells The body is a lot less likely to sacrifice
those just to kill off a virus. Then, herpes basically hangs out. Forever. Or at least, until the virus determines that it’s safe to re-enter the lytic phase
and reproduce — at which point, congratulations on your new
cold sore.

But it turns out, in a lot of people, herpes
may be… more of a frenemy than an outright foe. Herpesviruses certainly cause their fair share
of problems. The infections can be severe for immunocompromised
people, and they’re a leading cause of blindness
worldwide. They can also cause some cancers. But for the most part, herpesviruses have
evolved to keep their hosts alive and healthy enough to interact with
others and infect new hosts.

Chickenpox blisters and cold sores are the
exception — many herpesvirus infections are mild or totally
asymptomatic. Like, by the time we hit 35 years old, most
of us have been infected with the herpesvirus known as Epstein-Barr. And most of those who are infected have no
idea. That’s right: EBV causes most cases of mono,
but that miserable sore throat and fatigue doesn’t happen to most people who
get it. Lucky them. And weirdly, having herpes hanging out might
actually benefit our immune systems. But that’s a story for another episode. In the meantime, if you had chickenpox as
a youngster, at least now you know: you had the privilege of hosting one of the
most talented viruses around. Thank you for watching this episode of SciShow. Much like a herpesvirus infection, our patrons stick with us through thick and
thin. You’re always there for us, and you come
out in even more strength when things get bad. You help us make videos for everybody, even
those who cannot pay for them. We love you, even though we just compared
you to herpes.

If you want to get involved and help support
what we do here, check out patreon.com/scishow. [♪ OUTRO].

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