Which germs can cause pneumonia?

Bacteria and other germs can get into your lungs in tiny droplets of saliva (spit) or water that you have breathed in. But people sometimes develop pneumonia if something goes down the wrong way. In other words, if germs from their mouth and throat, bits of food, or stomach contents enter their airways. The medical term for this is aspiration pneumonia. Pneumonia is only rarely caused by germs that have traveled from another infected part of the body to the lungs in the bloodstream.

Bacterial pneumonia is most frequently caused by pneumococcal bacteria. But it is sometimes caused by other types of bacteria such as Hemophilus influenzae or Mycoplasma pneumoniae.

If pneumonia develops in a hospital or nursing home rather than at home, it is more likely to be caused by other bacteria such as staphylococcus, enterobacteria such as klebsiellae or escherichia coli (E. coli), or the “hospital bug” pseudomonas aeruginosa. These are often more difficult to treat, so there's a greater risk of complications such as lung abscesses, empyema (a build-up of pus in the lungs) or sepsis.

Pneumonia can also develop without bacteria being involved. Then it is usually caused by viruses – particularly those that can lead to infections of the airways, such as influenza (flu) viruses or RS (respiratory syncytial) viruses. But coronaviruses and the viruses that cause measles can also lead to pneumonia. The SARS-CoV-2 virus is one type of coronavirus. It can cause COVID-19, which can infect the lungs.

Pneumonia can also be caused by a fungal infection – for instance, by yeast and mold, or a fungus called pneumocystis jirovecii. But that is rare. Fungal infections of the lungs generally only develop in people with very weakened immune systems – as a result of AIDS, for example.

If the lungs are already infected with viruses, that makes it easier for bacteria to grow there too. When that happens, it is called a bacterial superinfection or a secondary bacterial infection. A superinfection can lead to bacterial pneumonia, or make viral pneumonia worse. For this reason, people who have viral pneumonia are generally given antibiotics. The antibiotics don't fight the viruses but they can prevent a bacterial infection from developing too.