What do the common cold, the flu, and AIDS have in common? They are all diseases caused by viruses, tiny microorganisms  that can pass from person to person. It is no wonder that when most people think about viruses, finding ways to  _ them is what is on people's minds.    Not everyone runs from the tiny disease carriers, though. In Cambridge, Massachusetts, scientists have discovered that some viruses can be helpful in an unusual way. They are putting viruses to work, teaching them of the world's smallest rechargeable batteries.   Viruses and batteries may seem like an unusual pair, but they are not so strange for engineer Angela Belcher, who first came up with idea. At the Massachusetts Institute of Technology in Cambridge, she and her team bring together different areas of science in new ways. In the case of the virus-built batteries, the scientists combine what they know about biology, technology and production techniques.   Belcher's team includes Paula Hammond, who helps put together the tiny batteries, and Yet-Ming Chiang, an expert on how to store energy in the form of a battery. Many batteries are already pretty small. However, every year, new electronic devices like personal music players or cellphones get smaller than the year before. As these devices become smaller, ordinary batteries won't be small enough to fit inside.    The ideal battery will store a lot more energy in a smaller package than ordinary batteries. Right now, Belcher's model battery, a metallic disk completely built by viruses, looks like a regular watch battery. But inside, its parts are very small--- so tiny that you can only see them with a powerful microscope.    How small are these battery parts? To get some idea of the size, pluck  one hair from your head. Place your hair on a piece of white paper and try to see how wide your hair is--- pretty thin, right? Although the width of each person's hair is a bit different, you could probably fit about 10 of these virus--- built battery parts, side by side, across one hair. These microbatteries may change the way we look at viruses. What do we know from the passage?
Belcher's team members are doubtful about her idea of the virus-built batteries. Ordinary batteries can't meet the needs of new electronic devices as time goes on. Engineers only need knowledge of biology to produce virus-built batteries. People can see virus-built battery parts with their eyes directly.