A free virus particle may be thought of as a packaging device for the virus’s genetic material. Every virus is a bunch of genes wrapped in a coat of proteins. In viruses, the genes are either made of DNA or RNA. The outer coat of a virus looks a little like a burr that sticks to your socks after walking in grass or weeds. In fact HA proteins, the exterior of a virus, are what stick to the cells in your nose, throat, or lungs. When the virus executes the first step, intrusion, the virus plunges into the cell and its protein spikes cling onto little yellow v-shaped structures (called receptors) on the outer wall of your cell (the cell membrane). The tip of the spike is like a key, and the receptor is like a lock; if the key fits the lock, the virus sticks to your cell. If it sticks, it then is able to enter your cell. Once the virus is inside your cell, it travels through main part of the cell, the cytoplasm, and heads toward the center of the cell, also known as the nucleus.

Once the virus has successfully breeched the outer wall of your cell, it starts the second phase, reproduction. When the virus goes into the nucleus, the viral genes (RNA/DNA) start to use your cell’s energy supply to reproduce. The chemicals called polymerases, that the virus entered with, is used to make thousands of copies of the viral genes. Then, the individual genes move out of the nucleus and back into the cytoplasm of your cell.

In the third stage, manifestation, the genetic material goes through the cytoplasm then out of the cell to a host a new cell. In the cytoplasm, the virus’s genes make lots of new viral proteins, including the protein spikes. The genes and the proteins move toward the cell membrane and form into thousands of new viruses. They need to break free of the cell they are in and go on to infect new cells so they push out against the membrane of the cell, forming little buds, trying to get free. Most of the budding viruses do get free; little chemical scissors cut them loose, allowing them to find new cells to infect. And so the same cycle begins again, in another cell. When you get a flu infection, in the first few days millions of new viruses infect millions of your cells until your immune system comes to the rescue and eliminates the infection.





http://www.infoplease.com/ce6/sci/A0861813.html
http://www.health.harvard.edu/flu-resource-center/virus/how-a-virus-infects-a-cell_3.htm