Viruses are very small particles with the sole purpose of making more of themselves. They are much smaller than our own cells, even smaller than bacteria.
Because they are so small, they don't have all the equipment inside that is necessary to make copies.
They are not much more than a blueprint for more viruses and a protective shell.
So how do they multiply? By finding cells to hijack. There are all kinds of viruses and they like to choose different cells to attack.
The flu virus targets the respiratory system: your nose, throat, and lungs.