Excerpt from an interview at:
http://www.geocities.com/CapeCanaveral/Galaxy/8152/robertbakker.html

S.B: What about extinction, you point to disease?

R.B: It was not an asteroid or comet, because it would have killed everything. I do point to disease. When big animals are spreading and mixing extinction occur. You can see this with elephants. One researcher just determined that African and Indian elephants make each other sick. When a new animal or plant is introduced to a habitat bad things happen. The biggest danger to native wildlife is foreign wildlife.

S.B: If so, how could we prove this?

R.B: There is a record of land bridges forming, and animals spreading. There, of course, is a record of extinction. Look at snails. They don’t spread very fast, and they do not become extinct. Land bridges were everywhere during the extinction, many species were spreading, and there were many diseases.

S.B: If this is so, then why did such a large percentage of sea life become extinct?

R.B: Often extinctions in the ocean occur at the same time as those on land. Then again, the ice age extinctions lost many big animals, but not many sea faring ones. At the end of the Cretaceous, though, oceans were changing and this was non related to the land extinctions caused by disease.

Bakker's book "Dinosaur Heresies" first mentioned this theory.