virii is a variant plural of the word 'virus'
It is important to note it is actually incorrect and the correct plural for 'virus' is 'viruses'
People use virii instead of viruses because
a) it looks 'better' or they want to appear of as smarter
b) they don't know it is incorrect
c) it sounds 'better' when said (vi-ree) compared to viruses (vi-rus-is)
If you use 'virii' people will announce 'your' mistake - use virii
Virii can often reproduce itself via email attachments.