I am curious about whether it's possible to embed virus in online video ( in the swf and/or flash format)?
Or is it possible to put a virus inside the html5
There always is a potential for malicious code to be distributed in any format or program, etc. Buffer overflows allow for arbitrary code to be executed. There are other ways to get in aswell. It would be hard pressed, but there always is a way since computers are finite machines and that limits their chance for absolute security.
As of late, Adobe has been criticized and been called "the pre-2002 Microsoft" for the flaws in Flash, so we might see more and more flaws with that and Reader.