Where do new infectious disease from?

Answer 1

Usually mutation. A virus is RNA based, and is somewhat sloppy with it's gene transcription. Most times a virus enters a body it picks up parts of the hosts genetic code. This causes virii to mutate at a much higher rate than other life forms. These mutations can completely change the way a virus manifests itself in the host. Aside from that, they can also jump species. Sometimes a virus will go largely unnoticed in the animal population, mutate at some point, and then become devistating to humans. Supposedly this is where AIDS comes from.