con- "with, together" + tangere "to touch"

1. (of a disease) spread from one person or organism to another by direct or indirect contact:

〈病気などが〉接触感染する, 伝染性［病］の(→ infectious); 〖通例be ～〗 〈人が〉(接触)伝染病にかかって

e.g. a contagious infection.

(of a person or animal) likely to transmit a disease by contact with other people or animals:

e.g. precautions are taken with anyone who seems contagious.

2. (of an emotion, feeling, or attitude) likely to spread to and affect others:

〈感情･態度･行動などが〉人に移りやすい, すぐに広まる, 影響の大きい.

e.g. her enthusiasm is contagious.

contagiously |kənˈtājəslē| adverb

contagiousness |kənˈtājəsnəs| noun

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 humor or optimism.