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.