What does it mean if a disease is described as contagious?

When a disease is described as contagious, it means that it can be easily transmitted from one individual to another, typically through direct contact or through the air. This characteristic indicates a high level of infectivity, meaning that the disease can spread quickly within a population. Understanding this term is important in public health as it helps in implementing necessary precautions, such as vaccination and quarantine measures, to control outbreaks. The other options describe characteristics that do not align with the definition of contagious diseases, such as being exclusive to animals, rare, or permanent, which further clarifies the unique aspect of how contagion operates in spreading disease among people.