Many respiratory diseases are caused by germs (viruses and bacteria) that spread from person-to-person when they are in close contact with others. These germs usually spread in respiratory droplets caused by coughing and sneezing; people also become infected when touching something with respiratory germs on it and then touching their mouth, nose or eyes. Other respiratory illnesses are caused by inhalation of infectious particles (bacteria and fungi) that can naturally exist in the environment (i.e. soil, water, etc.).
Some respiratory illnesses, such as influenza, can be prevented with a vaccine.