|MadSci Network: Virology|
Very simply viruses can only reproduce when they encounter a host cell. but cells can reproduce by themselves.
Viruses cannot make their own proteins so they "borrow" the abilities of a cell to do this for them. They infect a host cell by injecting their DNA into the cell. The virual DNA then takes over the cell and copies the viral DNA to make new viruses. Once there are too many new viruses inside the cell they burst out of the cell (killing it in the process). The new viruses then go on to infect more cells.
Cells reproduce by division (mitosis). The cell DNA is copied so there are two sets then the cell divides sharing out the DNA equally. The new cells have excactly the same DNA as the original cell. This process is constantly occurring in your body to grow organs, tissues etc and to repair wounds. Another type of cell division is meiosis which occurs to produce sex cells (eggs and sperm). This is a more complicated process which results in four cells but they only have half the original DNA. Then when a sperm meets an egg cell they join together to make a full set of DNA.
See this website for more information http://www.howstuffworks.com/cell.htm
Try the links in the MadSci Library for more information on Virology.