Define a contagious disease.

A contagious disease is specifically defined by its capacity to be transmitted easily from one individual to another, often through various means such as air, water, or direct contact. The key characteristic that sets contagious diseases apart is their ease of spread, meaning they can infect a large number of individuals in a relatively short timeframe. This is often due to the nature of the pathogens involved, which can survive outside a host and effectively gain entry into new hosts through everyday interactions.

While other options mention aspects related to diseases, such as the speed of development, incubation periods, or modes of transmission, these do not fully encapsulate the core definition of what constitutes a contagious disease. The aspect of easy transmission is pivotal, as it highlights the public health implications and the necessity for prevention measures to control outbreaks and protect community health.