Question: how do baterica and viruses infect the lungs?

Samuel Ellis answered on 9 Feb 2022:

A big question to answer, as there are all sorts of respiratory infections!

One thing to consider is where they come from. Viral infections typically are caught from someone else, from breathing in particles in the air or maybe touching a contaminated surface and then your face. Bacteria can also be transmitted like this, but also some bacterial lung infections occur with species which were already there in places in your nose and previously were not causing a problem until they got into the lungs.

How they infect also differs. Viruses have to get into our cells, for example SARS-CoV-2 does this using its spike protein to bind first. Once inside they hijack the cell to make more viral proteins to assemble many more viral particles, ready to burst out and spread to more cells.

Bacteria on the other hand are living cells and replicate on their own as long as they can find the nutrients they need and space to grow. They can colonise the lining of the airway for example, or stick in the mucus on the surface. Certain bacteria can also be invasive, forcing their way into the bloodstream and spreading to other body parts.

Most mild respiratory infections are in the upper airway, so nose and throat areas. Infections lower down in the lungs can be more serious, such as pneumonia. Serious symptoms are kind of side effects of the immune system trying to fight off the infection. For example, the air sacs in the lungs can get inflamed or even fill with fluid which makes it harder to breathe.

A final point, as well as bacteria and viruses, lung infections can also be caused by some fungal species too!