Democratic presidential nominee Hillary Clinton's immediate future on the campaign trail was in doubt Monday after it was disclosed she was battling a case of pneumonia. The announcement came hours after she fell ill Sunday during a Sept. 11 memorial in New York City, where aides had to help her to an awaiting vehicle. 

The 68-year-old's diagnosis is a common illness that every year affects millions of people, some of whom require hospital care. As the fall season approaches, the risk for catching pneumonia increases. 

What is pneumonia? According to the Centers for Disease Control, pneumonia is a lung infection, caused by viruses, bacteria and fungi. Although illnesses like influenza (the flu) and respiratory syncytial virus (RSV) — both of which have a higher rate of infection rate during the fall and winter months — are regularly associated with pneumonia, the National Heart, Lung, and Blood Institute reports the most common way adults catch pneumonia is from streptococcus bacteria, which accounts for up to one-third of the cases reported in the United States each year.

When a person’s lungs is infected by these viruses and bacteria, the immune system tries to block the spread of infection by sending attack cells. However, those cells can sometimes lead to air sacs in the lungs – known as alveoli – to become inflamed or filled with fluid and pus, which results in the symptoms of pneumonia.

What are the symptoms? There are a couple of factors that can determine the severity of pneumonia symptoms, including age, overall health and the specific type of pneumonia. The most common symptoms include coughing, fever, chills and shortness of breath, but people can also suffer from chest pains, headache, excessive sweating, clammy skin, loss of appetite and lower energy. Elderly people have been known to also experience confusion due to pneumonia. Viral pneumonia symptoms are often mistaken for influenza, because of the similar fever, dry cough, headache and muscle pain people experience. Whereas bacterial pneumonia symptoms tend to be more extreme, complete with body temperatures as high as 105 degrees Fahrenheit, heavy sweating, rapid breathing, confusion or delirium.

Who is most at risk? Anywhere between five million to 10 million people suffer from pneumonia in the United States each year, and more than one million people are hospitalized because of the illness, according to the New York Times. Although proper treatment for most cases of pneumonia results in successful recovery, the infection is still responsible for 40,000 to 70,000 deaths each year. The most commonly affected people are older adults — particularly the elderly who live in nursing homes or are already sick — very young children, pregnant women and people suffering from impaired immune systems.

How is it treated? Bacterial pneumonia can be treated with antibiotics, and most people suffering from the sickness show signs of improvement within one to three days. However, it may take more down time for people with viral pneumonia to recover since antibiotics do not fight viral infections. Drinking fluids and getting lots of rest is one of the best ways to help the body recover from pneumonia.