contagious

Origin:

late Middle English: from late Latin contagiosus, from contagio (see contagion)

Strictly, a contagious disease is one transmitted by physical contact, whereas an infectious one is transmitted via microorganisms in the air or water. In practice there is little or no difference in meaning between contagious and infectious when applied to disease or its spread. In figurative senses contagious may describe the spread of good things such as laughter and enthusiasm or bad ones such as violence or panic, whereas infectious usually refers to the spread of positive things, such as good humour or optimism.