Pneumonia is an infection of the lower respiratory tract caused by bacteria, viruses, or fungi. The infection can cause symptoms ranging from moderate to life-threatening. According to the Centers for Disease Control and Prevention (CDC), more than one million children and adults in the U.S. are admitted to a hospital for pneumonia each year. Worldwide, pneumonia kills more people than any other infectious disease, about 3.1 million people a year, and is the leading cause of death in children under 5 years old, according to the World Health Organization. Pneumonia can occur at any time of the year, but the greatest numbers of cases are seen seasonally during influenza (flu) season, typically late fall through early spring.
Although anyone can get pneumonia, it occurs most frequently in those who are very young or over the age of 65 or people with weakened immune systems (immune compromised). Pregnant women and people with an underlying condition such as cystic fibrosis or damaged lungs from smoking are also at higher risk for pneumonia. This also includes people who are on mechanical ventilation or chemotherapy or have received an organ donation. People with AIDS are extremely susceptible to pneumonia.
Bacteria, viruses, and fungi are microorganisms that can cause lower respiratory tract infections. Pneumonia is most often caused by a microorganism that has slipped past the body's immune defenses and taken up residence deep within the lungs in tiny structures called the alveolar sacs. Bacteria are the most common cause and viruses are the second most common cause.
These microorganisms may be spread to others through microscopic droplets of respiratory secretions that become airborne when an infected person coughs or sneezes. Airborne droplets can be directly inhaled by another person or settle on surfaces such as countertops, keyboards, and phone receivers in the vicinity. The microorganism is spread when someone else touches the contaminated surface and then touches his or her own eyes or mouth or other mucous membranes. Pneumonia can also come from the normal bacteria in saliva, food or vomit when a person chokes or gags and accidentally breathes in their own fluids.
It takes more than exposure to a potential pathogen to cause pneumonia. Microorganisms are a constant presence in the environment, and people encounter sources of infection on a daily basis. In most cases, the lungs can handle the assaults. The respiratory system has layers of immune defense starting with the mucous and hair-like structures that trap and remove many pathogens from airways. Pathogens that make it past these initial defenses are then subject to attack by an array of immune system cells that identify and engulf pathogens identified as foreign to the body. Pneumonia occurs when these defenses are weakened or damaged or when the invading pathogens are virulent enough to overcome or evade them.
A wide range of viruses, bacteria, and (less commonly) fungi can cause pneumonia, but the majority of cases are due to just a few of these. The most likely microorganism causing a specific case of pneumonia depends upon the age and health status of the affected person and to some degree on the time of year. Those with weakened immune systems and those who have traveled to specific regions of the world may develop pneumonia that is due to more unusual microorganisms.
The following terms that may be familiar describe some general kinds of pneumonia:
- Walking pneumonia refers to a mild form of the disease that typically does not require bed rest. It is often caused by a virus or Mycoplasma pneumoniae.
- Lobar pneumonia describes a form of the disease that affects one part (lobe) of a lung.
- Double pneumonia is an infection that affects both lungs.
- Aspiration pneumonia, or pneumonia caused by inhaling one's own gastric juices or food particles, can be more difficult to treat. The pathogens that cause the infection may be harder to identify in these cases, so health practitioners may need to do more testing in order to prescribe the correct antibiotic. People with gastroesophageal reflux disease or individuals who have trouble swallowing due to other conditions are more at risk for this type of pneumonia. Aspiration pneumonia is sometimes called anaerobic pneumonia because the bacteria that cause it are usually beneficial bacteria that live in our digestive tract and do not require oxygen to live.
The setting where pneumonia was acquired will determine the likely cause of the pneumonia, how to best prevent the spread of the infection, and guide the appropriate treatment.
- Community-acquired pneumonia—when a person becomes infected during daily activities outside the healthcare setting
- Hospital-acquired pneumonia—when an infection occurs, for example, after surgery while connected to a ventilator or in an intensive care unit
- Healthcare-associated pneumonia—when a person is infected while in a healthcare-associated setting, such as a nursing home or dialysis clinic
Hospital-acquired and healthcare-associated microorganisms are more likely to be resistant to first-line antimicrobials. Community-acquired pneumonia is more likely to be due to bacteria that are susceptible to commonly prescribed antibiotics or due to seasonal viruses for which antimicrobial agents are not the appropriate treatment.