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Also known as: Lower Respiratory Tract Infection; Community-Acquired Pneumonia; Hospital-Acquired Pneumonia; Healthcare-Associated Pneumonia; Walking pneumonia; Double pneumonia; Lobar pneumonia; Atypical Pneumonia

What is pneumonia?

Pneumonia is an infection of the lower respiratory tract, an invasion of lung tissues by microorganisms that can cause symptoms that range from moderate to life-threatening. According to the Centers for Disease Control and Prevention (CDC), more than 1 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, more than 4 million people a year, half of them under 5 years old.

Although anyone can get pneumonia, it is most common and potentially severe in those who are very young, over 65, immune compromised, or who have an underlying disease or condition that affects their lungs. This includes people who have HIV/AIDS or have had an organ transplant and those who are on chemotherapy, pregnant, in intensive care (ICU), on mechanical ventilation, have scarred or damaged lungs such as from smoking, or who have a lung disease such as cystic fibrosis.

Pneumonia can occur at any time, but the greatest numbers of cases are seen seasonally during the flu season, typically late fall through early spring. Infection of the lungs can occur in five different ways:

  • Microorganisms can move downward from the upper to lower respiratory tract.
  • Airborne droplets from a sneeze or cough can be inhaled.
  • Surfaces contaminated with mucus or other respiratory secretions can touched by hands and spread to mucous membranes.
  • Microorganisms in a person's own oral or gastric secretions can be aspirated into the lungs.
  • An infection may spread from a localized site in the body, through the blood and into the lungs.

It takes more than exposure to a potential disease-causing microorganism (pathogen) to get 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 take care of microbial assaults. The body has layers of defenses that include mechanical barriers, protective microorganisms (normal flora), and the immune system. Pneumonia occurs when these defenses are weakened or damaged and/or when the invading microorganisms are virulent enough to overcome or evade them.

A wide range of viruses, bacteria, and (less commonly) fungi or parasites 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 compromised immune systems and those who have traveled to specific regions of the world may develop pneumonia that is due to more unusual microorganisms.

Pneumonia may be grouped by the medical community into different categories. These distinctions are primarily useful in helping to determine the likely cause of the pneumonia, in determining how to best prevent the spread of the infection, and in guiding 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 establishment, 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.

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