Virii and Life

Virii (from old Latin plural of "virus") or "viruses" as used in modern standard English, are strange entities.  They blur the definition of alive for professional and amateur scientists alike.  While they possess certain functions that mimic life, it is generally thought they are in fact not alive.  Viruses exist solely to replicate.  Most viruses will find a host and utilize an innate mechanism of the host to "trick" the host into reproducing the virus instead of itself.
Recently, in the Republic of Korea ("South Korea"), scientists have made a remarkable breakthrough in the fight against cancer.  Using the adenovirus as a base, they inserted human genes and produced a hybrid virus that seems to attack cancer cells. 
The adenovirus, along with the rhinovirus, are are common causes of respiratory illnesses, such as the common cold and conjunctivitis.
Viruses are so small, that many infect only bacteria. 
Many viruses contain protection against harm in the form of an "envelope," making them difficult to eradicate (as opposed to kill).
They are little more than complex protein carrying cases for genetic material - either RNA or DNA. 
Other complex proteins causing illness are prions.  Prions are malformed proteins that are easily mistaken for a common protein, but cause an error in an organism's system.  A common example would be Bovine Spongiform Encephalitis - or "Mad Cow Disease."