Contagious diseases are characterized by:

Contagious diseases are primarily defined by their ability to be easily transmitted from one person to another. This means that when an infected individual comes into close proximity with other people, the pathogens causing the disease can be quickly spread through various means, such as direct contact or airborne particles. This transmission can occur in everyday situations, making it essential to understand and implement preventive measures to control outbreaks.

While it's true that certain contagious diseases can be passed through contaminated surfaces or have specific routes of transmission such as respiratory exposure, these factors are not universal characteristics. Additionally, while some contagious diseases may cause severe reactions in infected individuals, this is not a defining feature of contagion itself. The primary emphasis of contagious diseases is their transmissibility, which directly relates to public health efforts in managing and preventing infections.