Like if just one viral particle gets into your system, what’s more likely to happen: it proliferates undetected and cause an infection, or the immune system eliminates it before it can do so?
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Nothing happens really. A certain viral load (minimal infective dose I think is the more correct phrase) needs to be met usually. No idea if one virion can ever actually meet that though–pretty unlikely, but possible. Typically some part of the immune system will catch a few strays before they can do anything–be it mucus, hairs, t-cells, or what have you.
edit: clarification
There are some questions that have perfectly natural, real-world answers, but nobody knows the answer. This question is statistical and involves a complex, microscopic system. Scientists aren’t going to inject a single virion into a person one day at a time until they get sick – and short of doing that, there may be no way to know.
A single virion? Unlikely. A single prion? A slow and horrific death.
Technically speaking, it only takes 1 virion to get into a cell an cause an infection.
But it is 1 particle floating around in a body, it is unlikely to ever interact with the needed surface proteins to infiltrate a cell, it could just float around forever never interacting, or some active or passive part of the immune system catches it, or the proteins that make up the envelope disassociate and it’s no longer viable.
I like to think of a viral infection like World War 2, storming the beaches of Normandy. Sure, technically you could have 1 guy with all the action movie hero luck take out all of the German army on the beach, but in reality you need large numbers of soldiers, because inevitably some number will not make it. So the smaller the number of soldiers, the less likely they’ll be able to establish a beachhead.
Every virion has a small chance of successfully infecting a cell. The immune system will usually spot a cell acting strange or showing off the wrong proteins to “guard” cells and tell the cell to kill itself. A single virion could beat the odds, but it’s rare. Imagine that each one has a 1/100 (so 1%) chance of successful infection — one particle will be hard to entrench, but if you put in 1000 particles, you will likely get a few infected cells and have a baseline for the disease to start up.
I think norovirus takes something like 20 to have a good chance of succeeding at its life cycle, which is terrifying.
It may infect you, or it may not.
There is a certain probability a virus is successful in entering a host cell and reproducing.
One viron can infect you, though the odds aren’t high.
The other problem is if it takes a while to develop due to low numbers (start with 1 infected cell instead of a dozen) it gives the adaptive immune system time to mount a response.
Viruses work as numbers game. For each virion, there is a chance it will get to the right place, attach to the right cell, get taken inside the cell, actually start reproducing inside the cell, etc. Probably hundreds of steps, each of which can fail. Our built in non-specific immune system is also there at every stage trying to stop many of these steps. And in the end, even if this one lucky virion makes it into the cell, the cell might just randomly die. The cells lining your insides are dying and being shed constantly.
Studies of COVID-19 for example suggested that it could take tens of thousands or even over a million virus particles to cause an actual “infection”.
Let’s take the rhinovirus as an example. It’s one of the most common cold viruses. Someone with a cold rubs their snot on a door knob. You touch the door knob. Then you touch your lips. A minute later you lick your lips. Now the virus is in your mouth…but then what? If you swallow it, the virus will just be destroyed in your stomach. However, if somehow it gets into the air for a minute and you breath it in, then has a chance to infect. But as in the beginning of my answer, there are a hundred ways this could fail. It’s not impossible but it’s very very unlikely for a single virion to cause disease. And not because of the “immune system” necessarily. Just bad luck for the virus is enough to prevent infection.
However, if your question is more like “assuming a single virion successfully infects a cell, would the immune system still win out?” then the answer could be no, the “built in” immune system at this point has failed. Now it’s up to the slower, more specific version of your immune system to attack the virus. That can take several days as your immunes system slowly comes up with a specific plan to remove this exact virus. This is part of what makes you feel sick, your own immunes system kicking into high gear.
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