Cancer Research UK do a great job online keeping their content informative and engaging at the same time. My guess is that they have no trouble getting people to connect initially, as Cancer is unfortunately one of those things that affects most of us one way or another at some point in our lives. However, keeping that interest and encouraging people to donate money is something that they definitely have to work hard at.
I especially like their latest Do Me A Favour campaign, which previewed on their Facebook page a few days ago, but officially launched today, and uses Gary Lineker to send personalised video messages. By giving information on the person you want to send a video to, such as their name, age, where they are from and favourite football team, a personalised message is created and emailed to them reminding them to check for any early signs and linking them into the main website at the end (see example below).
Although I don’t think you always need to use a celebrity in viral campaigns for them to work, in this case I think using Gary Lineker (and his wife!) has worked really well and added enough interest for people to use it. They have also layered the supporting information well, with great visual tools which allow people to check the various early signs at different parts of the body, and tons of video content ranging from how to examine yourself, to interviews with people who have had various treatments.