GREGORY P. BISSON: Hello, and welcome back.
In this lecture, we're going to talk about specific viral pandemics
of the 20th century using the framework for understanding pandemics
that we developed in lecture 1.
So at the end of this lecture, the learner
will be able to name two pandemics of the 20th century.
List three factors that contributed to these viruses causing pandemics.
And compare and contrast these two pandemics in terms of the factors
that led to their rise.
In the last lecture, we talked about the fact
that it's not just the pathogen that leads to pandemics.
It's the pathogen's interactions with hosts and the environment
that really determines the burden of disease in the population.
And we discussed that new pathogens that are
spread via the respiratory route in populations
that have weak or no existing immunity,
and that occur in times or in settings where there
is easy facilitation of transmission--
for example, by travel from one population to another--
or lack of effective measures to deal with the virus
through medicine or preventative measures
also contributes to spread within a population.
We're going to use that framework now to discuss a couple of infamous pandemics
in the past 100 years.
The first pandemic we're going to talk about is influenza or "the flu."
Influenza or "the flu" is caused primarily by influenza A and influenza B viruses
which are spread by respiratory droplets,
which are droplets that emanate from us when we speak-- or worse, when we sneeze
or cough in public.
This causes a mild to severe illness characterized
by fever, sore throat, and chills.
And many of you have had the flu.
But most importantly, the flu can be fatal--
most frequently when it leads to severe forms of pneumonia.
So the important thing about flu viruses
is that we've been living with flu viruses for a very
long time but these viruses change.
Specifically, they intermittently evolve their genetics.
And sometimes this happens in small ways but it can also
occur in very large ways as well.
What this does is generate viruses that are either somewhat new or very new
in terms of their ability to infect and cause problems in hosts.
So for example, if one flu virus changed today
in a very large way in terms of its genes and the proteins it expresses,
that may seem like a new virus to the body.
In that we would have little or no immunity to it.
So although the flu is always around, the specific
viruses that cause flu are always changing.
And there are novel flu viruses with different host effects
that are continuously emerging.
So you can see that when you look at all
these characteristics, influenza is really a viral disease that
has pandemic potential.
An interesting thing about influenza viruses
is that they can infect and mix in a variety of animals other than humans,
including pigs and birds.
Pig and bird flu viruses are called swine
and avian influenza, respectively.
But typically the swine and avian flu viruses stick to their specific hosts.
In other words, while both bird and swine flu viruses can infect humans
and make people very sick, these viruses usually cannot be transmitted from
human to human.
But if several things happen, this can change.
For example, when humans and pigs or birds interact in large numbers, as
may happen on chicken or pig farms.
For example, you can have multiple viruses
infect the same chicken or human and combine to create a new virus.
For example, a human virus that is highly
transmissible from human to human can infect
a chicken that is, at the same time, infected with an avian influenza virus.
And then the human virus can combine with the avian virus
to make a brand new virus that has avian virulence characteristics
and is transmissible from person to person.
Needless to say, flu viruses that emerge from these sorts of interactions
can be very problematic for human beings.
And this is what happened to cause what was called the 1918 Spanish flu
pandemic.
So I wanted to go over why this is considered a pandemic.
But before I do that, I want to point out
that this is called Spanish flu not because we
think that it originated in Spain,
but I think because Spain was transparently reporting
their cases of it because they were not involved directly
at that point in World War I.
So they were being transparent with how much disease
was in their population and saying, hey, there's
this flu outbreak that's occurring.
At the time Spain was reporting cases, there
were also cases being noticed in the US and elsewhere.
So it doesn't mean that it was Spanish in origin,
but that's the name that stuck.
But this was a new virus with avian characteristics
that appeared to originate from birds and was very novel to humans,
meaning we as hosts had little to no immunity to it.
And to make matters worse, this specific virus turned out
to be extremely pathogenic causing very severe disease.
So from a viral standpoint, there were several things
that made it able to cause a pandemic.
But as I pointed out previously, it isn't usually about just the virus.
Spanish flu originated at a time of great social upheaval related
to World War I. And certainly mass travel contributed to its spread.
Thousands of troops traveled across international borders
and intermingled in very close quarters.
As you recall, flu is spread by respiratory droplets
which can easily pass from person to person when
people are this close together.
Also, patients with severe flu illness need medical support, including
hydration and, in some cases, oxygen. And health care
was severely disrupted by a need for these people
to participate in wartime activities.
In addition, this was also a time prior to modern virology and antibiotics
as well as vaccines for the flu.
So all of these things contributed to let this very lethal virus
ravage the globe.
There are several mysteries about Spanish flu
that made it a particularly terrible pandemic that I
wanted to briefly point out here.
First, as you can see from this graph showing the number of deaths
per thousand population in the United Kingdom in 1918 and 1919,
there's three waves of influenza.
It wasn't clear why there wasn't just one big wave.
But there were three different waves.
One in the summer, then a massive deadly wave in the fall, and then another
in the subsequent spring.
Why did it pause then reemerge with such a vengeance?
Which of the main determinants of pandemics contributed to these waves?
Did the virus change after first achieving some level of immunity
in the population?
Or was it some environmental phenomenon?
This still isn't clear.
And the second mystery I wanted to point out.
That although flu is typically more highly
fatal in the elderly and the very young, Spanish flu
was actually very pathogenic and very lethal in young individuals 20, 30,
and 40 years old.
Why was this the case?
Even after recreating the virus from deceased individuals frozen in Arctic
climates, this still remains unclear.
But regardless, by the time Spanish flu had run its course overall,
one third of the world's population or 500 million individuals became infected
and 50 million died.
Eventually the Spanish flu subsided.
But the importance of influenza viruses to pandemics is illustrated by the fact
that since then, there have been several other influenza pandemics.
Two of which I wanted to point out here.
The first of these occurred in 1957, called Asian influenza.
Like the Spanish flu, Asian influenza was an avian-derived influenza A virus,
which as before, was a new influenza virus to which few had immunity.
It, therefore, spread globally.
It just wasn't as pathogenic as Spanish flu
but still it wound up killing 1.1 million people worldwide
and over 100,000 people in the United States.
Fortunately, by this time, we had tools available to learn
about the flu virus and which we could use to develop vaccines.
So for this and other reasons perhaps, this virus had less of a global impact.
In 1968, another influenza virus pandemic occurred.
Again, the virus changed and adopted avian features
and eventually killed one to four million people worldwide.
And this virus still persists today.
So if we go back to our framework, here now we
can consider another different cause of a pandemic, which
is human immunodeficiency virus or HIV.
And I use this example because of the global importance of HIV/AIDS.
But also because, the way that HIV has spread
has been different than influenza.
Involving a different combination of pathogen, host,
and environmental factors.
HIV also emerged as a new virus and it appears it had a chimpanzee origin.
This was novel to humans.
Meaning, we had little pre-existing immunity to it.
HIV is also highly pathogenic in that, if untreated,
it leads to the acquired immunodeficiency syndrome or AIDS.
Which results from an immune system too weak to fight off infections.
But HIV does this typically only after a long asymptomatic period.
This means that people can walk around for many years
without knowing that they are infected and during this time they
can transmit the virus to others.
The transmission of HIV is by blood.
So inherently, it is less transmissible than influenza.
But the key point is that patients can be infected with HIV for years
and not know that they have it.
The behaviors that lead to HIV transmission
are behaviors like intravenous drug use and sexual relations.
And these behaviors can be very difficult to change.
For one, intravenous drug users typically
have addictions and need specific-- usually multifactorial-- interventions
that are not widely available.
And so it is difficult to intervene on this behavior
in an effective, widespread way.
And the things that drive individuals having sexual relations, particularly
in impoverished populations, include factors that
can be difficult to change as well,
such as young women having transactional sex
for money in poverty-stricken areas.
But nonetheless, individuals walking around not knowing that they had HIV
and spreading it by the bloodborne route, really led to HIV's ability
to become a pandemic.
Again, the environment, our society, did not have effective treatment or even
diagnostics for many years after HIV was being transmitted throughout the world.
And we did not have effective treatment available until the 1990s.
So decades after it was spreading.
And we still have no HIV vaccine.
So if you look at the number of AIDS-related deaths globally over time,
you can see at its peak it's causing about 2 million deaths per year.
So just a tremendous effect on individuals and societies globally.
But what's interesting is that you can also see that this is going down
and the burden of HIV is declining as interventions are being created.
We're altering our environment to reduce the number of AIDS-related deaths
by introduction of antiretroviral therapy
and other interventions in populations.
So to summarize, I gave examples of two different types of epidemics.
But they share characteristics of being dependent to some extent
on not only the virus, but the host's susceptibility to the virus,
the behaviors that facilitate the spread of that virus,
and the environment in which transmission is occurring.
A novel virus capable of respiratory spread
emerging during a unique point in history
combined to produce the highly lethal 1918 Spanish flu pandemic.
The HIV/AIDS pandemic has also been facilitated
by the emergence of a novel virus capable of being transmitted
during long asymptomatic periods, interacting
with behaviors and the environment.
Finally, the concepts learned in these two
lectures you can now hopefully use to contextualize
the information on SARS-CoV-2 and COVID-19, coming up later in the course.
Using the framework of host, pathogen, and environment,
you can now think about what factors have
led to global dissemination of this virus and disease.