Contagious diseases are diseases which are passed from person to person. They can be passed by direct contact or by bacteria in the air.
Some diseases are very dangerous and these can spread quickly, causing sickness and sometimes death. In the 14th century in Europe, a contagious diseases called "bubonic plague' or 'black death' killed millions of people. No one knew how it spread and they could not stop it. Today, a contagious disease like bubonic plague can be stopped by modern medicine, but at the time, nobody understood how diseases were spread or what caused them. Even this century there have been outbreaks of serious contagious diseases such as typhoid, yellow fever and cholera.
Doctors and scientist have studied these diseases can prevent them if medicine is available. Unfortunately, many countries are crowded and disease spreads quickly. When this happens this is called an 'epidemic'. Even today doctors and modern medicine sometimes cannot stop epidemics until many people have already died.
How are contagious diseases passed?