(CNN) Pneumonia is a respiratory infection that affects the lungs, resulting in inflammation of the alveoli, the air sacs that make up the bulk of the lungs and help us breathe.
Infection causes alveoli to fill with fluid or mucus, limiting the amount of oxygen entering the sacs and making it painful to breathe.
There are many types of pneumonia, caused by different forms of infection, and the effects on the body range from mild to severe. Children are vulnerable, with pneumonia being the single biggest cause of infectious death in children worldwide, according to the World Health Organization.
Pneumonia is most commonly caused by bacteria or viruses, but it can also be caused by fungi.
In the United States, the Centers for Disease Control and Prevention say common causes of viral pneumonia are "influenza and respiratory syncytial virus (RSV), and a common cause of bacterial pneumonia is Streptococcus pneumoniae (pneumococcus)."
Another common bacterial cause of pneumonia is infection with bacteria Haemophilus influenzae type b (Hib), which also causes meningitis. Vaccines are available against the bacterial forms of infection, as well as influenza, and can help protect individuals from infections that could lead to pneumonia.
According to the American Lung Association there are several telltale signs of pneumonia:
Less common pneumonia symptoms include:
Other less common symptoms can include nausea, vomiting, wheezing and joint and muscle pain, according to the UK's National Health Service.
It's easy to get dehydrated when you have pneumonia, because you're losing fluids when you sweat from the fever, and you're losing fluid because your lungs are inflamed, according to Dr. Eric De Jonge, director of Total Elder Care at Medstar Health and associate professor of medicine at Georgetown University Hospital.
"I can easily see how someone can be weakened by a mild pneumonia to the point where they're overheated and get presyncope, or a mild fainting spell, or even a full fainting spell from the dehydration," he said.
He tells his patients with mild pneumonia to stay home and not exert themselves because of how easy it is to get dehydrated. De Jonge advises patients with pneumonia to make an extra effort to consume more fluids than they do when they are well.
"I think if people are proud and try to maintain their daily work, or in this case the heavy exertion of a campaign, that it's likely to stress their system. They may not be aware how much pneumonia is taking out of them in terms of dehydration."
Pneumonia can be spread in multiple ways. The viruses and bacteria are typically contracted by people breathing them into their lungs and then spread through airborne droplets when people sneeze or cough. It can also spread through blood, according to the WHO.
Atypical pneumonia can be caused by several types of bacteria, including Legionella pneumophila, mycoplasma pneumoniae and Chlamydophila pneumoniae, and can be passed from person to person.
Aspiration pneumonia infections can occur if you inhale food, drink, vomit or harmful substances, such as smoke or chemicals, into your lungs.
Pneumonia can spread in hospital or other health care environments.
Older adults, children and people with chronic disease are at risk. A weakened immune system, particularly in children (often the result of malnutrition, undernourishment or other pre-existing illnesses), can increase the risk of infection.
In the United States, about 50,000 people die from pneumonia every year, according to the CDC, although many deaths could be prevented with vaccines or treatment.
Globally, pneumonia makes up 15% of all deaths among children under 5 years of age, with more than 920,000 deaths reported in 2015, according to WHO.
But adult and child deaths from pneumonia vary greatly by country. In the United States, there were 101 deaths from pneumonia for every million deaths between 2001 to 2010 and 214 deaths per million in the UK, according to the British Lung Foundation. In the same time period in South Africa, there were 1,168 pneumonia deaths per million.
Treatment depends on the type of infection and its severity. Mild infections can be treated at home with plenty of fluids, rest and by controlling fevers with medication, according to the American Lung Association.
Bacterial infections can be treated with a range of antibiotics, including azithromycin, amoxicillin and erythromycin. For viral infections, antivirals are sometimes prescribed. Clinton's physician announced that she is currently on antibiotics, meaning she must have a bacterial form of infection.
There are several vaccines that can be helpful in preventing diseases that cause pneumonia, including whooping cough, influenza, measles and pneumococcal disease. Improving respiratory health by quitting smoking will help lungs stave off infection.
Other ways lessen the risk of contracting the disease? Washing your hands after you use the bathroom, change a baby's diaper or blow your nose, and before and after handling or preparing food, along with generally maintaining good hygiene.