Infamy gained from being the YouTube user who has uploaded the most popular video of a flavour-of-the-month song. A good example is the user Trizzy66, whose YouTube channel remained fairly empty until she uploaded the sole video of Rebecca Black's "Friday".
A typical side-effect of youfamy is the almost absolute apathy of YouTube users towards the other videos in the youfamous person's library.
"Did you see Jenn's YouTube account? She had practically no views until she uploaded that Jazz track. Now she's got a million views on that video. She has a case of YouFamy."
"Yeah, but her other video still only has 5 views."