Clinton diagnosed with pneumonia, cancels campaign appearance

Hillary Clinton appeared to pass out while leaving a memorial service for the victims of September 11th. It's a video that's been viewed millions of times on social media and could have a real impact on the presidential race.

Over the past few weeks, Clinton has been dogged by coughing fits and questions about her health. The campaign says Clinton was diagnosed with pneumonia, but the video isn't doing her campaign any favors.

There are a lot of theories floating around about the state of Hillary's health. Some are gaining traction on social media in the heat of the presidential campaign, but one thing we know for sure is Clinton is being treated for pneumonia.

What exactly is pneumonia? How serious is it and how do doctors treat it?

Pneumonia is a lung infection. Bacteria can cause it, or a virus or even a fungus. It can cause coughing, fever, chest pain and trouble breathing.

"It is a very common condition. I have had many, many patients.. see it in children, young, middle-aged -- more common as people get into their 60s and older. Most people recover completely with no long-lasting effects," explained Dr. Moe Bell of the University of Arizona College of Medicine in Phoenix.

At the U of A College of Medicine, doctors use a lifelike medical mannequin to teach students how to treat illnesses like pneumonia by asking the right questions.

"How is your breathing? Do you have pain when you breathe? How are you feeling compared to earlier today or yesterday?" asked Bell.

The good news is pneumonia is highly treatable.

"Well over one million people per year are hospitalized with pneumonia in the United States and it is much more common than that several million people every year contract pneumonia and are treated as outpatients, usually with antibiotics," said Bell.

Most people recover in one to three weeks.

Who is at risk? Older people, children and people with chronic diseases.

How do you guard against catching pneumonia? Some simple steps: wash your hands, don't smoke, get a flu shot every year, because flu often leads to pneumonia.

If you're at special risk, there is a vaccine for pneumonia.