Modern viruses can be very self replicate by email themselves to others.
It is essential for the protection of everything you have on your computer that you run a good virus application. If you lend media to someone else to copy a file, write-protect it first so that it won't get inadvertently infected. Scan any file you download off the Internet with anti-virus software. If your computer has good virus infection, it will almost certainly never get a virus. If it somehow gets infected with a virus anyway, immediately shutdown your computer, and do not reboot it from the infected disk to prevent the virus from wreaking more damage.