Which germs can cause pneumonia?

The bacteria and other germs enter the lungs when tiny saliva or water droplets are inhaled (droplet infection). But people sometimes also become ill because they swallowed the wrong way. That means that germs from the mouth and throat, food residue or stomach content can enter the airways and lead to pneumonia. The medical term for this is aspiration pneumonia. Only rarely do the germs spread from another infected part of the body to the lungs via the blood.

Bacterial pneumonia is most frequently caused by bacteria called pneumococci. Other types of bacteria such as haemophilus influenzae or mycoplasma pneumoniae sometimes also cause the disease.

People who develop pneumonia in a hospital or nursing home rather than at home are at greater risk of other bacteria causing the infection. They include staphylococci, enterobacteria such as klebsiellae or escherichia coli (E. coli), or the “hospital bug” pseudomonas aeruginosa. These types of germs can lead to complications because they are often more difficult to treat.

Pneumonia rarely develops without bacteria being involved. If that happens, viruses might then be the cause, especially viruses that can lead to infections of the airways such as influenza or special viruses called RS (respiratory syncytial) viruses. But measles and coronaviruses can also cause pneumonia. The SARS-CoV-2 pathogen is one such coronavirus. It can cause COVID-19, which infects the lungs.

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

Bacteria can spread more easily if the airways are already infected by viruses. Doctors call that a bacterial superinfection or bacterial secondary infection. A superinfection can lead to bacterial pneumonia, or make viral pneumonia worse. That is why people who already have viral pneumonia are generally given antibiotics. They are used to prevent an additional bacterial infection rather than actually fight the viruses.