Pneumonia is an infection of the lungs, an invasion of lower respiratory tract by microrganisms that can cause a disease that ranges from mild to life-threatening. 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 another disease or condition that affects their lungs. This group of conditions includes people who have HIV/AIDS or have had an organ transplant and those who are on chemotherapy for cancer, are pregnant, in intensive care (ICU), on mechanical ventilation, have scarred or damaged lungs as occurs 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 in the late autumn through to early spring. Infection of the lungs can occur in five different ways:
- Microorganisms can move downwards from the breathing airways into the lungs
- Airborne droplets from a sneeze or cough can be inhaled
- Surfaces contaminated with mucus or other respiratory secretions when touched by hands can spread through the mouth or eyes
- Microorganisms in a person's own mouth or stomach can enter the lungs
- An infection may spread from another 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 always present in the environment, and we are exposed to infection daily. In most cases, the lungs can take care of these infections. The body has layers of defences that include mechanical barriers, protective microorganisms (normal flora), and the immune system. Pneumonia occurs when these defences are weakened or damaged and/or when the invading microorganisms are strong enough to overcome 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 to cause pneumonia will depend upon the age and health of the person and on the time of the year. Those with poor immune systems and those who have been traveling abroard may develop pneumonia that is due to more unusual microorganisms.
Pneumonia may be grouped by the medical community into different categories. These distinctions are most useful in finding a cause of the pneumonia, how best to prevent its spread, and in guiding treatment.
- Community-acquired pneumonia—when a person becomes infected during normal daily activities
- 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 from a healthcare establishment such as a nursing home or dialysis clinic
Hospital-acquired and healthcare-associated microorganisms are more likely to be resistant to first-line antibiotics where as 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.