First, if there was a chain letter for every virus on the internet, the servers across the country would be so clogged nothing would funciton. There are TONS of viruses out there. This is nothing new. If you get a virus alert, delete it. Don't forward it to anyone.
Viruses are nothing special.
By sending it to them, this implies that they don't realize the points I just detailed, so maybe you should send THIS to them, and not those virus alerts. Bottom line - it's not true. I have never heard of a company or any organization donating money to a charity based on how many people read a particular email, and if I ever DO, I will certainly inform you. Think about it - you get this email and send it to all your friends. In the American Cancer Society's case, they provided an AOL email address in the email.