Diseases caused by microorganisms are considered 'contagious' because the 'foe' microorganisms responsible for them can be transmitted from one infected individual to another, either directly (e.g., through touch, respiratory droplets) or indirectly (e.g., through contaminated surfaces, vectors). This implies that the pathogens are capable of surviving outside the host for a period and can successfully infect a new host, leading to the spread of the disease within a population. Contagiousness is a key characteristic of many infectious diseases.