If there were no atmosphere, the sky would be all black, except for the area directly around the sun, which you would not be able to look anywhere near because it would be so bright, as there would be no atmosphere to block its rays.
Contagion and contagiousness refers to how easily the spread of Genetic Disease is possible from one person to another. Other words for contagion include infection, infectiousness, transmission or transmissability. Contagiousness has nothing to do with genetics or inheriting diseases from parents.
Two of the most commonly used words in classifying disease are contagious disease and infectious disease. Let us differentiate one word from the other. Infectious disease is said to be the mother word of the terms such as contagious disease, communicable disease, and transmissible disease. Generally, infectious diseases are defined as the ability of a disease to enter a host, transmit the disease, and infect that person. An infectious disease can be sub-classified as contagious or non-contagious.