After Hillary Clinton appeared to collapse at the 9/11 anniversary ceremony on Sunday, her doctor confirmed she had been diagnosed with pneumonia prior to the event. Now, critics are questioning the Democratic presidential nominee’s health, her actions following the memorial, and the threat she may have posed to public health.

Clinton, 68, reportedly retreated to her daughter Chelsea’s Manhattan apartment, where she rested before emerging around noon and telling reporters, “I’m feeling great.” She greeted a young girl and put her hand on her shoulders to take a photo.

The former Secretary of State was diagnosed with pneumonia and put on antibiotics on Friday, her physician, Dr. Lisa R. Bardack, said in a statement.

Despite the criticism, several experts who have not treated Clinton said her reported symptoms and actions at the event suggest she had common bacterial pneumonia and did not threaten the health of those around her during the public appearance.

“It’s not too surprising that she has a pneumonia,” Dr. Frank Esper, pediatric infectious disease specialists at University Hospitals Cleveland Medical Center, one of those experts, told FoxNews.com. “She’s under a lot of stress, and the rigors of the campaign can lead you to be more susceptible to getting sickness.”

According to the National Institutes of Health, millions of Americans are diagnosed with pneumonia each year.

“Pneumonia is a very common infection— if you go around to the people you work with, you’ll have a hard time finding someone who hasn’t had it in the last 10 years,” Esper said. “Almost everybody has it several times through their life.”

Bardack neither revealed what type of pneumonia Clinton has, nor did she clarify whether it was a bacterial or viral infection of the lungs. Bacterial infections are not contagious, while viral infections are.

Viral respiratory infections are most common in late fall and early spring, so her illness is likely bacterial, Dr. Robert Kotloff, chairman of pulmonary medicine at Cleveland Clinic, told FoxNews.com. Bacterial pneumonia can be treated on an outpatient basis, and patients are advised to take it easy without further restrictions.

“There are more serious diagnoses that require hospitalization and, in some cases, care in an intensive care unit, but it doesn’t sound like this particular situation fits those criteria,” Kotloff said.

While some critics claim Clinton “fainted” while leaving the 9/11 event, it’s unlikely she actually lost consciousness, Dr. Amesh Adalja, infectious disease specialist at the University of Pittsburgh Medical Center, told FoxNews.com. Adalja said she may have been felt dizzy or felt faint, and, if that was the case, going to her daughter’s home rather than a medical facility was appropriate.

“It’s not uncommon for someone that’s being treated for pneumonia to have symptoms of fatigue and [feeling faint],” said Adalja, a member of the Infectious Disease Society of America's (IDSA) public health committee. “Sometimes those episodes are transient and will pass with a little bit of rest, laying down, drinking fluids, and there’s no need for further concern.”

Adalja added that Clinton’s physicians likely counseled her on which pneumonia symptoms to expect.

“She was not looking too short of breath, not wearing an oxygen tank, so she probably has a mild case of community acquired bacterial pneumonia,” said Adalja, adding that with antibiotics, patients usually have a “very robust recovery.”

However, it usually takes five to 10 days for an individual’s immune system to eradicate most viruses, after which point they are no longer infectious, Esper said. The immune system also wanes with age, which may make a patient Clinton’s age contagious for a slightly longer illness.

Depending on the specific virus that caused the pneumonia, a patient may be rundown for weeks and have a cough for months, Esper added.

“The lungs are extremely sensitive to infection, and it takes a lot to get well,” he said. “[Patients are] not coughing up infectious particles— their lungs have been scarred a little, and there’s an inflammatory response. They’re coughing up basically sterile material.”

While campaigning in Cleveland last week, Clinton had a coughing fit, which the campaign attributed to allergies. It's possible Clinton may have assumed that symptoms from an earlier viral infection were due to allergies, Dr. William Schaffner, an infectious disease expert at Vanderbilt University, told the Associated Press.

Clinton has stayed in the public eye since her diagnosis, but that doesn’t mean there should be concern that she’s spreading disease, experts like Adalja and Kotloff agree.

“For the overwhelming majority of bacterial organisms that cause pneumonia in adults, they’re not communicable from one individual to another,” Kotloff said. “In that way, I don’t think the child was put into jeopardy.”

It’s possible Clinton has what’s known as “walking pneumonia,” the common moniker applied to patients who do not need to be hospitalized, experts said.

“The majority of patients will just have a bad cough and need to go home, need chicken soup, need rest to give their immune system time to eradicate this germ,” Esper said, noting that, according to the ISDA , more than 1 million people over age 65 get pneumonia each year, which does not mean there are 65 million people hospitalized.

The Centers for Disease Control and Prevention (CDC) recommends the pneumococcal vaccine for all adults age 65 and older and it’s unknown whether Clinton received it. However, even if Clinton had been vaccinated, the formula protects only against the most common bacterial form of pneumonia, pneumococcal disease.

“There are a multitude of other organisms that cause pneumonia. More than 50 percent are caused by organisms other than pneumococcus,” Kotloff said. “Getting the vaccine does not dramatically reduce the likelihood of getting pneumococcal disease, it significantly reduces the likelihood of getting life-threatening complications from pneumococcus.”

The vaccination is also recommended for all children younger than 5 years old. Pneumonia is the leading cause of death among children under age 5, accounting for 15 percent of all child deaths in 2015, according to the World Health Organization.

An individual’s current health and health history do not make them more susceptible to pneumonia, Kotloff said.

“It can affect an otherwise perfectly healthy individual, whether they are elderly or even young adults. In general, stress and a heavy schedule that lead to exhaustion can certainly have an effect on the immune system,” Kotloff said. “Someone acquiring pneumonia does not in any way imply that they have an underlying health condition.”