Why do diseases often target the lungs?

Sally Le Page asked this infectous question to infectious disease researcher John Tregoning.

John - You need to think a bit more broadly. Anything that's inside you that faces the outside can get infected. We get gastric infections, we get GU infections infecting how you pee, but also how you breathe. And that's because those areas have to be a barrier, but they actually have to exchange things as well. Our lungs are essentially, if you imagine a sieve, they're very, very fine mesh that allows the air to go in and the carbon dioxide to come out. But at the same time, that's going to give space for bacteria and viruses to come through them. So the reason is that they are not just barriers, but they're exchange places. That's why you don't get infections on your skin, unless you get a cut. Then the bugs that live on your skin can get inside.

If everyone now takes a big breath in, you've just breathed in 10,000 bacteria and probably three fungi and a whole load of viruses. You're doing that all the time and yet you're not sick all the time. We have all these mechanisms that clear your lungs out, the sort of snot and the bogey that you have that sucks everything up and clears out. Unfortunately, the people who can't do that as well, so people have cystic fibrosis, have very sticky mucus in their lungs and the bacteria gets stuck in the lungs and can grow and kind of forms these colonies very similar to like what Huw might study in the bottom of the ocean. There are complex ecosystems in your lungs of the bacteria that live there. Related to that slightly but differently, is that on each tooth you have a completely different tooth microbiome. So every single tooth has a unique fingerprint of bacteria on it from the ones that kind of founded there and got dominance. Even when you clean your teeth, it just grows back to that kind of scuzzy stuff.

Sally - What stops them mixing from one tooth to another?

John - Because they're quite deep underneath the layers of liquid and the bacteria, they don't move between. But also it's so well established, what's in there already is forcing out anyone else. There's no kind of foothold in for other things.

Sally - I now don't know whether to feel good or bad about brushing my teeth. If I'm destroying these wonderful little ecosystems that are like the bottom of the Antarctic floor.

John - You should probably brush your teeth.

Sally - I should probably brush my teeth, yup. If you were a virus John, how would you spread?

John - If I was a virus I'd want to move from the nose to the nose because that's where sneezing would expel you furthest away. And you want to be able to go into that kind of that site so you can go to as many noses as quickly as possible. But you'd also want to be really stable so that you could stay on people's hands after they don't wash it when they get to the toilet and then they can put it on the surface and the Tube and the next person can come along and smear it into their own noses.

Sally - So basically a common cold. You'd be a common cold.

John - Common colds are pretty effective at moving around.