A Peer-to-peer Trojan Horse

A new “Trojan horse“ program called Phatbot is spreading across Windows computers connected to the Internet,
Phatbot can create invisible networks of up to 50 infected machines. This gives the hackers who wrote the program a highly efficient way to issue orders to the machines, in essence recruiting them into an underground hacker army.

Technology Review: MIT's Magazine of Innovation
This does not apply to Mac owners.