through a series of channels called bronchi,

and finally reaches little clusters of air sacs called alveoli.

There are some 600 million alveoli in the lungs,

adding up to a surface area of roughly 75 square meters—

the size of a tennis court.

These tiny sacs, only one cell thick, facilitate a crucial exchange:

allowing oxygen from the air we breathe into the bloodstream

and clearing out carbon dioxide.

Pneumonia wreaks havoc on this exchange.

Pneumonia is an infection of the alveoli that causes them to fill with fluid.

There are many different kinds of pathogens that can cause pneumonia.

The most common ones are viruses or bacteria.

These microscopic invaders enter the body via droplets either in the air we breathe,

or when we touch our eyes, noses,

or mouths after touching a contaminated surface.

Then, they face the respiratory tract's first line defense:

the mucociliary escalator.

The mucociliary escalator consists of mucus that traps invaders and tiny hairs

called cilia that carry the mucus toward the mouth, where it can be coughed out.

But some of these invaders may get past the mucociliary escalator

into the lungs, where they meet the alveoli.

Because alveoli serve as critical exchange points

between the blood and air from the outside world,

they have their own specialized types of white blood cells, or macrophages,

which defend against foreign organisms by enveloping and eating them.

When pathogens enter the lungs, the macrophages work to destroy them.

The immune system releases additional white blood cells in the alveoli to help.

As these immune cells fight the pathogens, they generate inflammation—

and fluid as a by-product of the inflammation.

When this fluid builds up,

it makes gas exchange inside the alveoli much more difficult.

As the level of carbon dioxide in the bloodstream begins to rise,

the body breathes more quickly to try to clear it out and get more oxygen in.

This rapid breathing is one of the most common symptoms of pneumonia.

The body also tries to force the fluid out of the alveoli through coughing.

Determining the cause of pneumonia can be difficult,

but once it is established, doctors can prescribe antibiotics,

which may include either antibacterial or antiviral treatments.

Treatment with antibiotics helps the body get the infection under control.

As the pathogen is cleared out,

the body gradually expels or absorbs fluid and dead cells.

The worst symptoms typically fade out in about a week,

though full recovery may take as long as a month.

Otherwise healthy adults can often manage pneumonia at home.

But for some groups, pneumonia can be a lot more severe,

requiring hospitalization and oxygen, artificial ventilation,

or other supportive measures while the body fights the infection.

Smoking damages the cilia,

making them less able to clear even the normal amount of mucus and secretions,

let alone the increased volume associated with pneumonia.

Genetic and autoimmune disorders

can make someone more susceptible to pathogens that can cause pneumonia.

Young children and the elderly also have impaired clearance

and weaker immune systems.

And if someone has viral pneumonia,

their risk of bacterial respiratory infection is higher.

Many of the deaths from pneumonia are due to lack of access to healthcare.

But sometimes, even with appropriate care,

the body enters a sustained fight against the infection it can't maintain,

activating inflammatory pathways throughout the body,

not just in the lungs.

This is actually a protective mechanism,

but after too long in this state organs start shutting down,

causing shock and sometimes death.

So how can we prevent pneumonia?

Eating well and getting enough sleep and exercise

helps your body fight off infections.

Vaccines can protect against common pneumonia-causing pathogens,

while washing your hands regularly helps prevent the spread of these pathogens—

and protect those most vulnerable to severe pneumonia.