QotW - why are some diseases infectious twice?

Why can we catch some diseases more than once, but others give us lifelong immunity?
01 October 2021



Listener Rick asks, "Why do we acquire lifelong immunity against some pathogens but not others?"


Sally Le Page asked author and infectious disease researcher at Imperial College, London, John Tregoning, to jog her memory...

John - The simplest answer is that some pathogens change and others don’t. Our immune system remembers what we have seen before and stops those pathogens infecting us again. Immune memory recognises molecules made by pathogens, mostly the ones on the outer surface that the pathogen uses to get into our cells. If these molecules change, then our immune system no longer recognises them, allowing the pathogen to infect us.

Sally - Seems like the immune memory is as bad at recognising people as me! If I’ve seen you before but now you’ve got a new haircut or are wearing a different pair of glasses, you might as well be a complete stranger.

John - Different types of pathogen use different methods to change how they appear to our immune system. Some viruses, like influenza and the coronavirus that causes COVID19 use a molecule called RNA to store their genetic information. When the virus replicates it makes copies of that RNA and mistakes occur which lead to changes in the structure of the molecules on the outside of the virus. Some bacteria also change the way they appear. The bacteria coat themselves in sugar to hide from the immune system. Each bacteria can make several different sugars and they can replace the one on their surface with a new one, improving their chance of not being seen.

Sally - The sneaky things! It’s almost like these bacteria don’t WANT to be spotted by our immune systems...

John - The pathogens to which we acquire lifelong immunity are more stable – they don’t change their surface proteins, so our immune systems can recognise them each time we see them.

Sally - So the next time you catch a cold, feel sorry for your immune system which is having the very awkward experience of not recognising someone they’ve already met. If the thought of that social anxiety is making you want to leave everyone behind and just live in space, this next question from Daniel might make you think again…

Daniel - If a crew on a mission to Mars has a death on board and the body was released into space, would that body ever decay?


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