Why is pneumonia so common and severe?

Breathing exposes our lungs to microbes. We take a breath every few seconds throughout our lifetime. With every breath, we bring about half of a liter of the outside world into our lungs. Air contains a variety of contaminants including bacteria, viruses, and fungi. In addition, inhaled air passes through our mouth and nose on the way to the lungs. These upper airways are colonized by bacteria even in healthy people and are sources of exposure for many pathogens.

Microbes and microbial products such as toxins interfere with lung cells, so they must be eliminated when they get into the lungs. The lungs have a variety of defenses good at eliminating small numbers of innocuous microbes that land there. However, when larger numbers of microbes or particularly virulent microbes get in, then inflammatory responses become necessary. Inflammation helps kill microbes, but it makes breathing more difficult and less effective.

Physicians have good tools to fight lung infections. Vaccines prevent some microbes from causing lung infections, and antibiotics cure patients of many types of pneumonia. Despite these useful tools, lung infections remain a critical public health concern. No vaccines are available for many microbes that infect the lungs. People with compromised immune functions are difficult or impossible to effectively vaccinate even if vaccines are available. Microbes develop resistance to previously effective antibiotics. Some microbes are not susceptible to any known drugs. Even when successful, the eradication of microbes is sometimes insufficient to save patients succumbing to inflammatory injury such as the acute respiratory distress syndrome.

Novel tools must be developed, which requires new knowledge. We need a better understanding of how lungs are exposed to microbes, so we can decrease such exposure. We need a better understanding of how innate immune cells and proteins eliminate microbes in the lungs, so we can augment such elimination. We need a better understanding of acute inflammatory responses in the lungs, so we can more effectively and specifically control them. We need a better understanding of how lung cells protect themselves and their surrounding tissues from inflammatory damage, so we can enhance this protection.