If you've been debating whether to get a flu shot, you should watch this beautifully terrifying animation of a virus taking over a cell and sending millions of its progeny off to spread the infection.
A single virus particle can turn an infected cell into a factory capable of producing millions more virus particles, which it sends out to infect other cells. To fight that onslaught, your immune system has to be able to destroy virus particles before they can spread and infect healthy cells. The good news is that usually, your immune system is on top of things.
But in order to kill viruses and other pathogens, the cells of your immune system have to recognise them as intruders. Every time your immune system encounters a pathogen, it creates cells called B and T cells specially designed to target that intruder if it ever shows up again. That means your immune system can respond much more quickly and effectively to things it's been exposed to before.
That's why vaccines are so important: they expose your immune system to a dead or weakened version of a virus, so it can create the T and B cells to fight the virus, but you don't actually have to get sick. Vaccine are basically combat training for your immune system. Without that training, your immune system might not be able to recognise and fight off a viral infection quickly enough.