We all get the flu from what is called a virus. A virus isn't really dead or alive, but all viruses need a living call to survive. That's why any living things-plants, animals, even bacteria-can get a virus. A virus common to humans is the flu virus.
Viruses have information inside that allows it to keep making copies of itself. This information is called RNA. You've probably heard about DNA somewhere before. DNA is basically the instructions for building molecules that our bodies need. When those instructions are copied, they go through a "spell-check," so any errors in the replication are corrected. Viruses contain RNA instead of DNA, and RNA doesn't go through a "spell-check" for errors. When the influenza virus enters our bodies with a lot of errors, our immune system doesn't recognize it. By the time our immune system finally responds to the virus, we may already be sick.
This is a computer image of what DNA looks like. The sprial-like shape is called a double helix. This image shows only a tiny fraction of the entire molecule, but shown at this scale the image would be 970 meters high! This computer image was used with permission from Juergen Suehnel.