ELI5: Why isn’t our immune systems completely immune to things like the common cold?

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As I understand it, the virus mutates and develops into different strains, which differ in very small, basically cosmetic ways but make it impossible for the body’s immune system to identify it and respond correctly, so it treats it like an entirely new disease. It’s kind of like if you own a shop. Someone keeps shoplifting from you. You eventually catch them and put up a poster with their face on it so all your staff know to not let them in, but then they come back with comedy glasses and a fake moustache and no one recognises them at first. Eventually your staff realise, but not until after they steal a few things. This is an incredibly oversimplified explanation, I’m sure someone else can do a better job.


I HAVE A LESSON PLAN JUST FOR THIS!! Let’s pretend this smiley emoji is a common cold virus 🙂 that gets you sick. Once your immune system knows how to fight it off, it can recognize it, and 🙂 ever shows its face around you again, it’ll beat 🙂 up! So you dont get sick again! But uh oh! You got your sister sick, and the cold changes a little while shes sick 😐. And your sister gets your brother sick, and it changes a little in him 😮. And then mom gets sick and it changes a little in her 😯. Now your mom exposes you to the same cold that’s been going around the house; the one _you_ started. Let’s compare it with the one you had. 😯🙂 Not very similar are they? Maybe you and I, with our big brains, can see that it’s the same virus pulling a different face, but your immune system can’t. So you get sick again. _But Professor Beauty, the flu is ALSO a virus, and it’s a lot like a cold, and we can get a flu shot so we dont get sick! Why can’t we get a Cold shot??_ Another great question! The Cold virus is SUPER good at changing itself, so by the time scientists made a vaccine for it, that version of the cold may not be a big threat anymore (as threatening as a cold is to the general population) The flu is a bigger threat, and it’s _pretty_ good at changing itself, but not that good. The success of the flu vaccine is largely dependent on a bunch of scientists working year round to predict what the flu will “look like” (in keeping with our emoji model) and which versions will be the most dangerous/infectious. Which is why sometimes you get the flu shot and still get sick! If scientists predict that the flu strain in a certain year will look like one of these 🙂😐😮, and predicts that these 2 🙂😐 are going to be the heavy hitters, theyll distribute 2 vaccines, one that covers just the MOST likely candidate 🙂 and one that covers BOTH 🙂😐 for the at risk population. If you get the 🙂, but then you’re exposed to 😮? You’re not immune to that one, and you’ll get sick. At the end of the day, flu strains are similar enough that getting a vaccine, even for the wrong strain, can make the sickness from a different strain a little easier. (Not a question anyone asked, but an answer you got anyway) And also, back to the original question, sometimes you get a disease like measles that just, wipes out your whole “Diseases I Know How To Fight Database” and you gotta start over. 🙁


First of all the Common Cold isn’t something you can be immune to because it’s a bunch of symptoms and not a pathogen. The Common Cold is caused by different viruses. Most of them are Rhinoviruses but some are Coronaviruses and some others I can’t remember. The vast majority of colds are caused by Rhinoviruses and here’s the catch……………….. there’s around 200 different strains of them. There’s also different strains of the other viruses but almost all that cause the Common Cold are Rhinoviruses. That’s a lot of viruses and their strains you would have to develop an immunity to. I doubt you catch that many colds. And finally, on top of that, they have the nasty little habit of mutating. So, by the time you get around to contracting the same strain again it’s probably mutated so much that your immune system no longer recognises it.


Three main reasons. One:Just as our immune systems are evolving to fight diseases better, diseases evolve to avoid the immune system better. One of the most tenacious things about the common cold is that viruses of that type can evolve very quickly to avoid being destroyed by our immune system. This also makes it very difficult if not impossible to design a vaccine against them. Two: We have enough humans globally interconnected enough that diseases can continue finding humans that haven’t experienced the disease(and begun developing immune defenses) yet for a LONG time. Imagine how long it would take for a cold that started in south America to travel and infect people in Mongolia. Three: your immune system doesn’t remember diseases forever. It depends on a lot of factors but you can be reinfected with a strain of cold that you’ve experienced before several years after your infection.


The main reason is that “the common cold” is not a specific virus it’s a series of symptoms. It’s a bunch of things your body does when being infected by some pathogens. It’s a bit like saying “why aren’t we immune to fever or diarrhea”. We know of more than 200 virus than cause common cold, so even if you catch 2 colds per year and get lifelong immunity, you can still get new ones all your life. In addition every year we add 1-2% of fresh humans (babies). This provides a pool without immunity at all where this viruses can survive even if a lot of adults are immune.