Just recently something happened: people using Twitter started seeing messages saying "Don’t Click". Most people are curious so they clicked. The link had an iframe with some hidden code that sent a Twitter message using your account, telling others not to click. Technically this is a virus because it propagated through a network.
The result was that a message saying "Don’t Click" quickly spread through Twitter. No harm was done. But it could have been harmful. Whoever came up with the idea managed to get lots of people to click on an unknown link, and it could have had malicious code on it.
Twitter says they’ve fixed the problem that made this possible. But it highlights a problem with Twitter, that people are seeing links they don’t understand and are clicking on them.
More info here.