Final answer:
Legionella causes disease in humans because of limited transmission methods and high virulence, which means it requires specific conditions to spread and can cause severe infections when those conditions are met. It is not effectively transmitted person to person, hence it is not a widely communicable pathogen.
Step-by-step explanation:
Legionella is able to cause disease in humans but is not an effective pathogen in terms of spread due to Limited transmission methods, and relatively high virulence once infection takes place. Legionella pneumophila, the bacterium that causes Legionnaires' disease, thrives in aquatic environments, particularly in warm water systems like those in air conditioning units, and is transmitted to humans through contaminated aerosols. Unlike other infectious diseases, Legionnaires' disease is not spread from person to person, which limits its communicability. The nature of its transmission and the need for specific environmental conditions prevent Legionella from being a highly communicable pathogen.
Meanwhile, the pathogen's virulence can cause severe disease, especially in individuals with compromised immune systems or preexisting health conditions. The ability of Legionella to invade and multiply within human cells, combined with the fact that infection usually requires inhaling contaminated water droplets, explains why the disease is more severe but less likely to be transmitted between individuals.