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.
Once inside the host cell, they set up shop, using the cells' raw material to create new viruses.
Viruses are copied and copied and copied until the host cell can't hold them anymore.
It bursts open, releasing thousands of new viruses which then find new cells to infect. This goes on and on until your body can fight back against the attack.
Immune System to the Rescue
After a while, your body will figure out it is being attacked and decides it's not going to take it anymore. Your body's immune system produces virus fighting weapons called antibodies.
Antibodies are tailor made to defend against specific attackers. They float around inside your body and once they come into contact with their enemy they turn into goo and surround the virus, preventing any more cells from being infected.