The revelation that Hillary Clinton has pneumonia is putting a spotlight on an illness that affects more than three million Americans each year. Health Reporter Erin Billups takes a look at pneumonia, and what a diagnosis could mean.

Hillary Clinton's wobbly entrance into her van, after abruptly leaving the September 11th commemoration ceremony, caused a firestorm of speculation Sunday.

"There’s lots of things that could cause those symptoms," says Dr. Benjamin Stein, Lead Physician at Northwell Go Health Urgent Care.

We now know Clinton is suffering from pneumonia. Her campaign says she was diagnosed Friday, is on antibiotics, and probably should have been at home resting.

Urgent Care doctor Benjamin Stein says without knowing the exact type of pneumonia Clinton has, it's hard to determine its severity.

"There are lots of different causes for pneumonia and lots of different types of pneumonia. The most common causes are either viral or bacterial, but there are fungal types and parasitic types of pneumonia," he says.

After leaving the ceremony, Clinton spent about 90 minutes at her daughter's home, later emerging with a smile and a wave for onlookers before heading to a check-up.

In a statement released late Sunday, the doctor said Clinton was diagnosed with pneumonia after she was treated for a prolonged allergy-related cough, and that Clinton became "overheated and dehydrated" at Sunday's event.

"If you’re not drinking enough fluids, being dehydrated can result in fainting. Some of the antibiotics, the medication we give you can cause side effects that can result in fainting. Some antibiotics can cause diarrhea, it's also a common issue. The dehydration that's a result of losing fluids can predispose someone to faint," explains Stein.

In general, pneumonia isn't as contagious as strep throat, the flu or a cold. And depending on the patient's health, other underlying illnesses and even age, the outcomes are wide ranging.

"If untreated it can become serious and patients can die. But with treatment these days, and with a patient who doesn't have any underlying immunocompromisation patients usually do very well," says Stein.

A pneumonia diagnosis is arguably vague, underscoring the need for more information.

It seems Clinton's campaign agrees, announcing Monday it will release more details on her health soon.