This is about how society treats people who are "different." A lot of people think they are "sick" and "wrong" and want nothing to do with them. In the middle of the song, David acts out a skit where he is being abused by his mother (not in the radio version). Finally, he builds up enough courage and hits her right back. This is used to symbolize how "mother society" abuses "her" children for not being the way "she" thinks they should. "she" keeps doing it until "her" children get sick of it and are so angry with her and themselves they go out and get revenge for it, most likely by killing the people that hurt them ("here it comes, get ready to die!") A good example of this would be the school shootings that go on around the country. Getting "Down With The Sickness" means accepting people for who they are so we can all function better.