Virus, worms, trojan horse, spyware and adware are called malware, short for malicious software. Virus, worms and trojan horses are commonly sent as email attachments.
Apple (Mac OS) computers are not impacted by the following information, but Computers with Windows OS are. If an attachment file name ends with exe, pif, bat, bas, cmd, com, eml, inf, js, lnk, msi, scr, or vbs, do NOT click on it! That will make the program execute (open up and run). Even if you know the name of the person that it came from, they could be infected and not know it. Be especially careful if it's from someone you don't know.
To make matters worse, malware writers can disguise their work to look harmless because of a weakness in the Windows Operating System. By default "Show File Extensions" is turned off. You can't see the real file extension unless this is turned on. If you don't have protection that's up to date you may already be infected.
Both the Windows operating system and the Mac Operating System can be infected with a virus through Microsoft files (Word/Excel) containing MACROS.
by Sandy Szabo, the Digital Surgeon.