But when it comes to some diseases, the way the body defends itself can have unintended consequences.

In the case of pneumonia, the immune system's response can be lethal, earning it the nickname

the 'captain of the men of death.'

Pneumonia doesn't just refer to a single virus or bacteria.

It's a condition that can actually be caused by a number of different bacteria, viruses,

or even fungi, the most common being the bacteria streptococcus pneumoniae, who will be our main bad guy today.

So when we're talking about pneumonia, we're really referring to something that is happening to our lungs.

Hi, my name's Keith Klugman.

I'm the director of the pneumonia programs at the Bill & Melinda Gates Foundation.

I'm particularly fascinated by pneumonia and have been working to try and prevent pneumonia in poor children all of my life.

So pneumonia is an infection of the lungs and it is a very common infection.

The problem is that the lungs are thought to be sterile...in fact the lungs aren't sterile.

But generally there's very few bugs in your lungs. The problem is that our

mouth and our nose are actually filled with bacteria and viruses and these can sometimes

get down into the lungs.

We're constantly being exposed to the bacteria and viruses that cause pneumonia like Streptococcus pneumoniae.

These pathogens can live in your upper respiratory tract without you even knowing it.

And that's because, for most people,

the immune system should be able to step in and stop them in their tracks, leading to immunity.

So you become immune to the bugs that you encounter when you're very young and by about

five years of age, you're pretty much immune.

But, for children under 5, the immune system is weaker, leaving the body susceptible to infection.

And it's not just children who are at risk.

Then, unfortunately, pneumonia risk comes back again in the elderly.

"Who is elderly?" you might say.

Generally defined as people older than the speaker.

So, for me, it's 65 and older…

So pneumonia is a disease of the extremes of life, the very young and the very old.

Now, it's not only young children and older adults, anyone with a compromised immune system is at risk.

With the immune system unable to mount a defense in the respiratory tract, the bacteria are able to pass into the lungs.

Once there, the immune system will still try to defend the body.

But...

Unfortunately, they have a lot of side effects when this response occurs.

Macrophages first try to fight off the infection, but they can become overwhelmed, triggering the release of cytokines.

These cytokines lead to inflammation of the lungs, causing air sacs called alveoli to fill with fluid.

Yeah.

So alveoli are air sacs in the lungs that are where the exchange of oxygen and carbon dioxide take place.

And it's these alveoli that get filled with the fluid that's come from your bloodstream.

Now, there's no particular reason for red cells to get out of the bloodstream but white

cells that are included in the bloodstream are very potent cells able to kill bacteria

and they move in from the bloodstream out into the tissues at the site of infection.

But, filling the alveoli with fluid does more than just fight off the infection.

That's because, basically, your lung sacs are filling with pus.

The lungs are there and exquisitely designed for oxygen to pass from the air into your

bloodstream...And so unfortunately, instead of having nice, clear air pockets where this

exchange can take place, if they're filled with fluid, then the exchange of oxygen and

the release of carbon dioxide, which you breathe out, also is impaired.

This causes difficulty breathing as well as a whole host of symptoms that vary greatly in severity.

The effects of walking or atypical pneumonia can be so mild, someone might not know even know they have it.

In other cases, the infection can lead to death.

But, while pneumonia is still a disease that kills millions of people worldwide, there

isn't a good reason why it should still be so lethal.

The main driver of mortality from pneumonia, is access to treatment or lack thereof.

So, in fact, there are vaccines that work to protect from both bacterial and viral pneumonias.

In fact, the most successful vaccine for prevention of pneumonia is a bacterial vaccine.

So pneumonia deaths really shouldn't occur.

It's preventable, as I mentioned, by vaccines, also treatable by antibiotics.

Because of this, pneumonia deaths are much more common in poorer countries with fewer resources.

Which is why Dr. Klugman is working on ways to develop more effective treatments in these areas of the world.

But pneumonia deaths can still happen in places like the United States, especially if something

were to lead to people having compromised immune systems.

That could happen from contracting other illnesses, some of which are entirely preventable.

So, as you know, there have been outbreaks of measles recently in the United States.

And globally, measles has not been eradicated.

One of the major problems with measles is that it diminishes your immune response.

And kids who have measles can get bacterial and viral pneumonias after their measles so,

in fact, pneumonia is a major killer following measles exposure.

So a vaccine, there's a very potent vaccine for measles.

And measles prevention is a very important way of preventing deaths from pneumonia.