If you do feel compelled to pass on a virus warning, do your homework first, and make sure your email includes Web addresses of documented evidence of the authenticity of the virus. As the state of the art advances in software development, and operating systems become more sophisticated, viruses are getting more sophisticated as well. Stealth viruses hide themselves from virus scanners. Yes, antivirus programs are just programs, and they're not immune from viruses, either, although they take precautions to avoid or detect infection. Polymorphic viruses actually change themselves to hide better from virus scanners.
Polymorphic viruses are able to mutate so that they look different in every host they infect.
Sometimes this is done through encryption technology, using a different random key each time, and sometimes through actual mutation, where the code of the virus changes. Hostile applets are Java programs that load in a Web page. I recommend turning Java and JavaScript off in your email program. Damage is grotesquely overestimated, and the viruses are sensationalized. Companies that write antivirus software want to scare you so you buy their software.