We have such fantastic immune systems--better than any computerized system man could design.  Did you know that you have something in your immune system that "interferes with" foreign, harmful invaders like viruses?  Appropriately, it is called interferon.
Learn how interferon works as an essential component of your immune system at this website:  http://www.immunityfacts.com/index.html   There is also a great section called "Understanding the Immune System".  If you have any questions contact me.