Now let's talk about influenza.
Influenza is a highly contagious,
viral, respiratory infection
that infects the nose,
the throat, and the lungs.
It's caused by influenza viruses that are
common in late autumn to early spring.
And children younger than 5 years,
especially those younger than 2,
are at a high risk of serious
complications from influenza.
How do we prevent influenza?
That's the best protection.
Influenza is an RNA virus with a
roughly spherical lipid envelope.
The outside of the virus capsule
is covered with spikes of proteins
that are involved in the
viral docking, endocytosis,
and fusion of the viral
membrane to the host cell.
Here is the N protein or neuraminidase.
Inside the envelope is the capsid,
and that's a protein shell
that contains the genetic
information of the virus.
At the core, there are
about 8 strands of RNA
that are patiently awaiting their
release into the host cell.
The H protein, or the hemagglutinin, is
another spiky glycoprotein on the surface
that helps it bind with the host cell.
Influenza is a virus that actually
has hundreds of different strains.
The virus mutates frequently,
but the strains are classified into
1 of 3 main categories: A, B, or C.
Now, influenza C does not cause
serious illness, so in this lecture,
we're primarily focusing on
influenza A and influenza B.
You can remember this.
That is awful.
It is responsible for frequent,
usually annual, local outbreaks
or local epidemics that vary in
intensity every 2 to 3 years.
Then occasionally, it will cause pandemics.
Influenza A causes more severe symptoms
than influenza B or influenza C.
Influenza A viruses can be
broken down into subtypes
depending on the genes that
makeup the surface proteins.
Over the course of a flu season,
different types, A and B,
and the subtypes of influenza A
circulate around and cause illness.
Within the type A influenza viruses,
there are subtypes based on
their H and N antigens.
This allows for multiple different
combinations or strains
that circulate within the
community each flu season.
Antibodies to 1 subtype of H or N antigen
do not react with the other
type of H or N antigens.
a patient can get sick with
multiple strains in the same flu season.
There are 3 antigenic subtypes of
the H antigen: H1, H2, and H3.
And there are 2 antigenic subtypes
of the N antigen: N1 and N2.
Now we'll talk about the B viruses.
These are a little bit better.
You can remember them.
And the outbreaks happen
about every 4 years.
Patients with influenza
type B typically have
milder disease than those
with influenza type A.
And the other type, type C, that we're
not really going to cover much,
doesn't cause serious illness
in the general population.
So here we are with Type A.
Remember, it's awful and
responsible for annual outbreaks.
All influenza A viruses are further broken
down into those H and N subtypes.
So any influenza virus that's
described as an H with a number,
and then an N with a number, such as H1N1
or H3N2, is a type of influenza A virus.
Other combinations have been
found to infect other species,
such as birds and pigs, but they have
not caused widespread human infection.
There are 16 H subtypes and 9 N subtypes,
but only 3 of these combinations
have actually caused
highly contagious illness in humans.
The 3 combinations that cause almost
all outbreaks of flu in humans
are the H1N1, the H2N2, and the H3N2.
Type B strains, remember, do not cause
as severe of symptoms as type A.
And over on the right, type C, those don't
really cause any serious symptoms.
The influenza virus is
transmitted through the spread
of infected respiratory
droplets that are aerosolized,
sprayed out into the air, by
coughing, sneezing, or talking.
It can also be spread with
direct contact with fomites,
and these are phones, doorknobs,
countertops, shopping carts.
And I find it interesting when
patients come in during flu season
and they always say, "Where
do you think I got the flu?"
And, "Where could I have possibly
come in contact with the flu?"
And my answer is, "It's everywhere."
It's in the air of the people around you
who are coughing and sneezing
and spraying their droplets.
And then it is settling on to all
of the objects in the environment
just waiting to be touched by you.
This is going to enter the patient's
tracheal bronchial epithelium,
where the virus is going
to begin to replicate.
The patient will shed
the most of their virus
in the first 48-72 hours after
they've contracted the virus,
and this is when they may not
be feeling super symptomatic.
These patients are still going to
school, they're still going to work,
and they are spreading the flu around.
Patients will be contagious for about
5 days after their symptoms begin.