What is the definition of the word infectious?

Answer 1

The standard definitions is "(of a disease or disease-causing organism) likely to be transmitted to people, organisms, etc., through the environment."
It has come to mean " likely to spread or influence others in a rapid manner."

Answer 2

The word "infectious" refers to the ability of a disease or agent to spread from one person to another through direct or indirect contact. It can also describe something that is likely to evoke a similar reaction or behavior in others.