The cause of most diseases come from virus and bacteria just like; the SARS, the common cold, the Bubonic plague, the Ebola virus, and the Flu. All of those diseases come from a bacterial or virus that can spread in many ways.
The spread of those diseases are all contagious. During the flu season or any virus out- break it is good to keep form inhaling other peoples sneezes or coughs, also washing your hands after touching objects in public, is a good way