1. Of or relating to contagion.
2. Transmissible by direct or indirect contact; communicable: a contagious disease.
3. Capable of transmitting disease; carrying a disease: stayed at home until he was no longer contagious.
4. Spreading or tending to spread from one to another; infectious: a contagious smile.

1. Capable of being transmitted by direct or indirect contact, as an infectious disease.
2. Bearing contagion, as a person or animal with an infectious disease that is contagious.

Our Living Language  : A contagious disease is one that can be transmitted from one living being to another through direct or indirect contact. Thus the flu, which can be transmitted by coughing, and cholera, which is often acquired by drinking contaminated water, are contagious diseases. Although infectious is also used to refer to such diseases, it has a slightly different meaning in that it refers to diseases caused by infectious agents—agents such as viruses and bacteria that are not normally present in the body and can cause an infection. While the notion of contagiousness goes back to ancient times, the idea of infectious diseases is more modern, coming from the germ theory of disease, which was not proposed until the later nineteenth century. Contagious and infectious are also used to refer to people who have communicable diseases at a stage at which transmission to others is likely.