The only way to catch a virus is to run an infected program. When an infected program is run, the virus is also run, which allows the virus to replicate by attaching itself to other programs. There are some viruses that can crash computers and even damage files and some that do nothing but replicate. You can not get a virus by viewing a web page or reading email. It has been reported that security problems in Java may allow viruses to exist in applets, but I have heard of no attacks by these viruses.
Software is available to detect and remove viruses.
It is important to remember that dangerous viruses are actually quite rare. If the people who are being warned don't know much about viruses they will not recognize it as a hoax and will perpetuate the hoax. The hoax spreads like a virus without ever infecting a single computer. A massive collection of HTML and other documents available through the internet. Your Mileage May Vary.