Movie seen around 2010, about the virus at the prom, a lot of blood, in some places disgusting, but with humor.
The beginning and end are stylized as a cartoon.
Cabin Fever 2: Spring Fever (2009)?
A high school prom faces a deadly threat: a flesh-eating virus that spreads via a popular brand of bottled water.
The intro starts with live action and becomes cartoonish:
Found with the Google query
movie virus prom site:imdb.com/title.