M.L.
If there is a link and you click on it, you can very likely get a virus, or if you open up a letter with an image that can carry a virus. There are several types of viruses, like a Trojan Horse that will allow people to hack into your accounts.
People will also forward chain letters and spread other's addresses to lots of people on their contact list. So, people you do not know (some of them spammers, even if it's just a Mary Kay lady) are given access to your email address through forwarded email. Internet Scambusters says, "If a spammer gets a hold of one of those, do you think they won't grab every address in the message? (We've seen one case in which a message had been forwarded so many times as attachments that it included over 1,100 addresses!) Internet Scambusters, Issue 41, January 18, 2001. Hoaxbusters says that some spammers are deliberately starting chain letters and hoaxes to gather email addresses.