|MadSci Network: Virology|
There are many different types of viruses which can cause a "cold". Some of the most common ones are the various Rhinoviruses, Adenoviruses and Respiratory Syncytial Viruses. The human influenza virus causes the flu, which can be mistaken for a bad cold.
Most of these viruses are thought to be able to be transmitted from a surface, such as a doorknob or keyboard for about 3 to 6 hours. But none of these viruses goes directly through the human skin into the blood. They must get to a mucous membrane for infection. You can touch the dirtiest keyboard in the world and not get infected with a cold virus unless you also touch your eyes or the inside of your nose. This is why washing hands often is important for reducing the spread of colds and flus.
We are taught to cover our mouth and nose with our hand when we cough or sneeze. This does help prevent directly infecting another person by coughing right into their face but it also increases the chances for transmitting the viruses to another hand during a handshake, or to a doorknob or keyboard or book or whatever... It would probably be better if we were taught to cough into a tissue, or our shirtsleeve, or to wash our hands after we cough into them.
Try the links in the MadSci Library for more information on Virology.