Source: http://www.kaspersky.com/about/news/virus/2012/Oracle_Java_s... (Chrome uses its own PDF system, not Adobe Reader, so it's not even close to the threat level of Java).

Java has had multiple drive-by exploits discovered in just the past few months, which by definition don't require confirmation.

I was inaccurate, however. Java is almost certainly the cause of most exploits, but I daresay that user inexperience (to put it kindly) is probably the source of most infections. Touchè.